For some reason, my game just acts like the mod doesn't even exist, no matter what I do the shulker preview doesn't appear.
I wanted to switch to the dev version so that I could provide a more detailed log, but then the game just refuses to start and crashes.
Crash report and the latest log:
latest.log

---- Minecraft Crash Report ----
// Hi. I'm Minecraft, and I'm a crashaholic.

Time: 3/25/21 1:39 PM
Description: Initializing game

java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by 'shulkerboxtooltip'!
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:53)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:36)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient.start(EntrypointClient.java:32)
at net.minecraft.class_310.(class_310.java:437)
at net.minecraft.client.main.Main.main(Main.java:177)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.fabricmc.loader.game.MinecraftGameProvider.launch(MinecraftGameProvider.java:226)
at net.fabricmc.loader.launch.knot.Knot.launch(Knot.java:146)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:28)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/item/Items
at com.misterpemodder.shulkerboxtooltip.impl.ShulkerBoxTooltip.(ShulkerBoxTooltip.java:50)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at net.fabricmc.loader.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:45)
at net.fabricmc.loader.EntrypointStorage$NewEntry.create(EntrypointStorage.java:112)
at net.fabricmc.loader.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:99)
at net.fabricmc.loader.EntrypointStorage.lambda$getEntrypointContainers$1(EntrypointStorage.java:184)
at net.fabricmc.loader.EntrypointStorage$$Lambda$84/1537772520.get(Unknown Source)
at net.fabricmc.loader.entrypoint.EntrypointContainerImpl.getEntrypoint(EntrypointContainerImpl.java:37)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:50)
... 11 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.item.Items
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at net.fabricmc.loader.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:168)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 21 more

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Thread: Render thread
Stacktrace:
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:53)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:36)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient.start(EntrypointClient.java:32)
at net.minecraft.class_310.(class_310.java:437)

-- Initialization --
Details:
Stacktrace:
at net.minecraft.client.main.Main.main(Main.java:177)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.fabricmc.loader.game.MinecraftGameProvider.launch(MinecraftGameProvider.java:226)
at net.fabricmc.loader.launch.knot.Knot.launch(Knot.java:146)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:28)

-- System Details --
Details:
Minecraft Version: 1.16.5
Minecraft Version ID: 1.16.5
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 1299396616 bytes (1239 MB) / 1946157056 bytes (1856 MB) up to 5368709120 bytes (5120 MB)
CPUs: 4
JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx5G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
Fabric Mods:
advancedchat: AdvancedChat 2.1.2
advancednbttooltip: Advanced NBT Tooltips 1.4.0+build.3-1.16.5
anvil_crushing_recipes: Anvil Crushing Recipes 1.0+MC1.16.2-1.16.5
anvil_fix: AnvilFix 1.2.1-1.16
appleskin: AppleSkin 1.0.11
architectury: Architectury 1.10.138
armorhotswap: ArmorHotswap 1.2
autoconfig1u: Auto Config v1 Updated 3.3.1
baritone: Baritone 1.6.3
bedrock_breaking: Bedrock Breaking 1.0.2
betterpingdisplay: Better Ping Display 1.1
blue_endless_jankson: jankson 1.2.0
cameraoverhaul: Camera Overhaul 1.0.1-fabric-1.16
chat_heads: Chat Heads 0.2.1+1.16.2
cleancut: CleanCut mc1.16-2.1-fabric
cloth-basic-math: Cloth Basic Math 0.5.1
cloth-client-events-v0: Cloth Client Events v0 1.5.47
cloth-config2: Cloth Config v4 4.10.13
cmdkeybind: Command Macros 1.5.2-1.16
colormatic: Colormatic 2.2.8+mc.1.16.3
com_github_kaptainwutax_biomeutils: BiomeUtils 590f697a2ccb6c6bdba8e2fea891a25ace75c947
com_github_kaptainwutax_chunkrandomreversal: ChunkRandomReversal 209eefb8ed2bd097e3c55d3934ba508b664443da
com_github_kaptainwutax_featureutils: FeatureUtils 25f73f26289a65a314cd66badc3c433d7f8c37b0
com_github_kaptainwutax_latticg: LattiCG 38f0b3d33e15ad2e6ce9ddb1f588e2b9a8c96174
com_github_kaptainwutax_mathutils: MathUtils 6c2d50eacad0241ff76119e6e703b70bac4b4bce
com_github_kaptainwutax_seedutils: SeedUtils 8e310c079346fa55077fd1786b50dae0d2025336
com_moandjiezana_toml_toml4j: toml4j 0.7.2
com_squareup_moshi_moshi: moshi 1.9.2
com_squareup_okio_okio: okio 1.16.0
commandblocksurvival: Command Block Survival 1.0.0
compostable-rottenflesh: Compostable Rotten Flesh 1.0.0
cotton: Cotton 1.1.0
cotton-cauldron: Cotton Cauldron 1.1.0
cotton-commons: Cotton Commons 1.0.4
cotton-config: Cotton Config 1.0.0-rc.7
cotton-datapack: Cotton Datapack 1.0.7
cotton-logging: Cotton Logging 1.0.0-rc.4
cotton-player-events: Cotton Player Events 1.0.2
craftable_god_apple: Craftable God Apple 1.0.0
craftpresence: CraftPresence 1.7.6
crawl: Crawl 0.8.2
crowdin-translate: CrowdinTranslate 1.2
dynamicfps: Dynamic FPS 2.0.1
dynamicsoundfilters: Dynamic Sound Filters 1.2.0+1.16.5
easiervillagertrading: EasierVillagerTrading 1.16.5-fabric0.30.0-1.5.4
fabric: Fabric API 0.30.3+1.16
fabric-api-base: Fabric API Base 0.2.1+9354966b7d
fabric-biome-api-v1: Fabric Biome API (v1) 3.1.1+ca58154a7d
fabric-blockrenderlayer-v1: Fabric BlockRenderLayer Registration (v1) 1.1.5+ca58154a7d
fabric-command-api-v1: Fabric Command API (v1) 1.0.10+ca58154a7d
fabric-commands-v0: Fabric Commands (v0) 0.2.2+ca58154a7d
fabric-containers-v0: Fabric Containers (v0) 0.1.11+9354966b7d
fabric-content-registries-v0: Fabric Content Registries (v0) 0.2.1+ca58154a7d
fabric-crash-report-info-v1: Fabric Crash Report Info (v1) 0.1.3+ca58154a7d
fabric-dimensions-v1: fabric-dimensions-v1 2.0.6+9354966b7d
fabric-entity-events-v1: Fabric Entity Events (v1) 1.0.3+ca58154a7d
fabric-events-interaction-v0: Fabric Events Interaction (v0) 0.4.2+ca58154a7d
fabric-events-lifecycle-v0: Fabric Events Lifecycle (v0) 0.2.1+ca58154a7d
fabric-game-rule-api-v1: Fabric Game Rule API (v1) 1.0.6+ca58154a7d
fabric-item-api-v1: Fabric Item API (v1) 1.2.1+ca58154a7d
fabric-item-groups-v0: Fabric Item Groups (v0) 0.2.3+ca58154a7d
fabric-key-binding-api-v1: Fabric Key Binding API (v1) 1.0.4+9354966b7d
fabric-keybindings-v0: Fabric Key Bindings (v0) 0.2.1+ca58154a7d
fabric-language-kotlin: Fabric Language Kotlin 1.5.0+kotlin.1.4.31
fabric-lifecycle-events-v1: Fabric Lifecycle Events (v1) 1.2.1+ca58154a7d
fabric-loot-tables-v1: Fabric Loot Tables (v1) 1.0.2+ca58154a7d
fabric-mining-levels-v0: Fabric Mining Levels (v0) 0.1.3+ca58154a7d
fabric-models-v0: Fabric Models (v0) 0.2.1+ca58154a7d
fabric-networking-api-v1: Fabric Networking API (v1) 1.0.1+ca58154a7d
fabric-networking-blockentity-v0: Fabric Networking Block Entity (v0) 0.2.8+ca58154a7d
fabric-networking-v0: Fabric Networking (v0) 0.3.2+ca58154a7d
fabric-object-builder-api-v1: Fabric Object Builder API (v1) 1.9.4+9354966b7d
fabric-object-builders-v0: Fabric Object Builders (v0) 0.7.2+ca58154a7d
fabric-particles-v1: Fabric Particles (v1) 0.2.4+ca58154a7d
fabric-registry-sync-v0: Fabric Registry Sync (v0) 0.7.4+ca58154a7d
fabric-renderer-api-v1: Fabric Renderer API (v1) 0.4.1+ca58154a7d
fabric-renderer-indigo: Fabric Renderer - Indigo 0.4.4+ca58154a7d
fabric-renderer-registries-v1: Fabric Renderer Registries (v1) 2.2.1+ca58154a7d
fabric-rendering-data-attachment-v1: Fabric Rendering Data Attachment (v1) 0.1.5+ca58154a7d
fabric-rendering-fluids-v1: Fabric Rendering Fluids (v1) 0.1.13+ca58154a7d
fabric-rendering-v0: Fabric Rendering (v0) 1.1.2+ca58154a7d
fabric-rendering-v1: Fabric Rendering (v1) 1.5.1+ca58154a7d
fabric-resource-loader-v0: Fabric Resource Loader (v0) 0.4.2+ca58154a7d
fabric-screen-api-v1: Fabric Screen API (v1) 1.0.0+c045166c7d
fabric-screen-handler-api-v1: Fabric Screen Handler API (v1) 1.1.5+9354966b7d
fabric-structure-api-v1: Fabric Structure API (v1) 1.1.4+ca58154a7d
fabric-tag-extensions-v0: Fabric Tag Extensions (v0) 1.1.1+ca58154a7d
fabric-textures-v0: Fabric Textures (v0) 1.0.6+ca58154a7d
fabric-tool-attribute-api-v1: Fabric Tool Attribute API (v1) 1.2.6+ca58154a7d
fabricloader: Fabric Loader 0.11.2
fallingleaves: Falling Leaves 1.5.0+1.16.5
fatxporbs: Fat Experience Orbs 0.0.9
fiber: fiber 0.23.0-2
fiber2cloth: Fiber To Cloth 3.0.1
flytre_lib: FlytreLib 0.2.8
food_highlight: FoodHighlight 1.0.1
gatewayer: Gatewayer 1.0.0
gbfabrictools: GBfabrictools 1.3.2+1.16.4
horsestatsvanilla: Horse Stats Vanilla 4.1.6
infinitemusic: Infinite Music 1.3.0
infinityfix: Infinity Fix 1.1.0
inventoryhud: Inventory HUD + 3.2.2
inventorysorter: Inventory Sorter 1.7.7-1.16
jankson: Jankson 3.0.1+j1.2.0
java: Java HotSpot(TM) 64-Bit Server VM 8
jumploader: Jumploader 2.1.3
kubejs: KubeJS 1605.3.6-build.177
kyrptconfig: Kytpt Config 1.1.6-1.16
lambdynlights: LambDynamicLights 1.3.4+1.16
libgui: LibGui 3.3.5+1.16.5
lithium: Lithium 0.6.4
mcwifipnp: Minecraft WLAN Plug-n-Play 1.2.8
mesh: Mesh 0.6.1-alpha.21+1.16.4
mesh_api: Mesh API 0.6.1-alpha.21+1.16.4
minecraft: Minecraft 1.16.5
misc_tab: MiscTab 1.0.0
mm: Manningham Mills 2.1
mobblindness: Mob Blindness 1.0.0
modmenu: Mod Menu 1.16.7
modupdater: ModUpdater 1.1.11+1.16.2-rc2
orderly: Orderly 2.0.0-alpha.10-1.16
org_aperlambda_lambdajcommon: lambdajcommon 1.8.1
org_jetbrains_kotlin_kotlin-reflect: kotlin-reflect 1.4.31
org_jetbrains_kotlin_kotlin-stdlib: kotlin-stdlib 1.4.31
org_jetbrains_kotlin_kotlin-stdlib-jdk7: kotlin-stdlib-jdk7 1.4.31
org_jetbrains_kotlin_kotlin-stdlib-jdk8: kotlin-stdlib-jdk8 1.4.31
org_jetbrains_kotlinx_kotlinx-coroutines-core-jvm: kotlinx-coroutines-core-jvm 1.4.2
org_jetbrains_kotlinx_kotlinx-coroutines-jdk8: kotlinx-coroutines-jdk8 1.4.2
org_jetbrains_kotlinx_kotlinx-serialization-core-jvm: kotlinx-serialization-core-jvm 1.1.0
org_jetbrains_kotlinx_kotlinx-serialization-json-jvm: kotlinx-serialization-json-jvm 1.1.0
paradoxconfig: Paradox Config 0.4.0-beta
phosphor: Phosphor 0.7.1+build.11
quickshulker: Quick Shulker 1.1.12-1.16
recipeconditions: Fabric Recipe Conditions 0.3.0
replaymod: Replay Mod 1.16.4-2.5.2
rhino: Rhino 1.7.13-build.14
riptide_fix: Riptide Fix 1.2.0
roughlyenoughitems: Roughly Enough Items 5.10.181
roughlyenoughitems-api: REI (API) 5.10.181
roughlyenoughitems-default-plugin: REI (Default Plugin) 5.10.181
roughlyenoughitems-runtime: REI (Runtime) 5.10.181
roughlyenoughresources: Roughly Enough Resources 2.1.3
sectionsign: SectionSign 1.0.2
seedcracker: Seed Cracker 0.2.3-beta
shulkerboxtooltip: Shulker Box Tooltip 2.3.5+1.16.5
shulkerutils: shulkerutils 1.0.1-1.16
slotlock: SlotLock 1.0.1-BETA
sodium: Sodium 0.1.0
spruceui: SpruceUI 2.0.4+1.16
stitchedsnow: Stiched Snow 1.0.1
tellme: TellMe 0.9.2
torohealth: ToroHealth Damage Indicators 1.16.4-fabric-4
transparent: Transparent 2.1.0
verticalredstone: Vertical Redstone 1.2.0
villagerhelper: Villager Helper 1.3.3
vistas: Vistas 1.2.4
voxelmap: VoxelMap 1.10.15
whospetisthis: Who's Pet Is This 1.0.0
winwonders: Window Wonders 0.1.2
worldedit: WorldEdit 7.2.3+5680-135941e
worldtime: WorldTime 1.16.5-fabric0.30.0-1.3.1
wurst: Wurst Client v7.14-MC1.16.5
Launched Version: fabric-loader-0.11.2-1.16.5
Backend library: LWJGL version 3.2.2 build 10
Backend API: NO CONTEXT
GL Caps:
Using VBOs: Yes
Is Modded: Definitely; Client brand changed to 'fabric'
Type: Client (map_client.txt)
CPU:
#@!@# Game crashed! Crash report saved to: #@!@# C:\Users\User\AppData\Roaming.minecraft\crash-reports\crash-2021-03-25_13.39.00-client.txt

I was able to fix this issue by deleting my configuration file entirely.

It was probably caused by the enablePreview setting in the mod's config that was set to false

The preview is enabled. I'm not that braindead at least to disable that. Ticket remains open.

By chance, do you have the server integration option turned off ?

Turning on server integration fixes it, but I don't think I should need server integration in singleplayer?

Right. Does this issue occur on the latest release of the mod ?

I just updated to the newest version 1.16.5 that you posted a few days ago.
Considering that not much changed, this kinda surprised me, but on the newest version, this issue no longer occurs.

Yeah I fixed a bug where previews would not display when server integration was disabled in this release