Hello. When the game loads up the minecraft:air block could not load its model.

[13:03:27] [Client thread/ERROR] [FML]: Exception loading model for variant minecraft:air#inventory, normal location exception:
net.minecraftforge.client.model.ModelLoaderRegistry$LoaderException: Exception loading model minecraft:item/air with loader VanillaLoader.INSTANCE, skipping
at net.minecraftforge.client.model.ModelLoaderRegistry.getModel(ModelLoaderRegistry.java:161) ~[ModelLoaderRegistry.class:?]
at net.minecraftforge.client.model.ModelLoader.func_177590_d(ModelLoader.java:302) ~[ModelLoader.class:?]
at net.minecraft.client.renderer.block.model.ModelBakery.func_177577_b(ModelBakery.java:175) ~[cgb.class:?]
at net.minecraftforge.client.model.ModelLoader.func_177570_a(ModelLoader.java:151) ~[ModelLoader.class:?]
at net.minecraft.client.renderer.block.model.ModelManager.func_110549_a(ModelManager.java:28) [cgc.class:?]
at net.minecraft.client.resources.SimpleReloadableResourceManager.func_110542_a(SimpleReloadableResourceManager.java:121) [cev.class:?]
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:513) [bib.class:?]
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:378) [bib.class:?]
at net.minecraft.client.main.Main.main(SourceFile:123) [Main.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_211]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_211]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_211]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_211]
at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:196) [NewLaunch.jar:?]
at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:231) [NewLaunch.jar:?]
at org.multimc.EntryPoint.listen(EntryPoint.java:143) [NewLaunch.jar:?]
at org.multimc.EntryPoint.main(EntryPoint.java:34) [NewLaunch.jar:?]
Caused by: java.io.FileNotFoundException: minecraft:models/item/air.json
at net.minecraft.client.resources.FallbackResourceManager.func_110536_a(FallbackResourceManager.java:69) ~[cei.class:?]
at net.minecraft.client.resources.SimpleReloadableResourceManager.func_110536_a(SimpleReloadableResourceManager.java:65) ~[cev.class:?]
at net.minecraft.client.renderer.block.model.ModelBakery.func_177594_c(ModelBakery.java:334) ~[cgb.class:?]
at net.minecraftforge.client.model.ModelLoader.access$1400(ModelLoader.java:115) ~[ModelLoader.class:?]
at net.minecraftforge.client.model.ModelLoader$VanillaLoader.loadModel(ModelLoader.java:861) ~[ModelLoader$VanillaLoader.class:?]
at net.minecraftforge.client.model.ModelLoaderRegistry.getModel(ModelLoaderRegistry.java:157) ~[ModelLoaderRegistry.class:?]
... 22 more
[13:03:27] [Client thread/ERROR] [FML]: Exception loading model for variant minecraft:air#inventory, blockstate location exception:
net.minecraftforge.client.model.ModelLoaderRegistry$LoaderException: Exception loading model minecraft:air#inventory with loader VariantLoader.INSTANCE, skipping
at net.minecraftforge.client.model.ModelLoaderRegistry.getModel(ModelLoaderRegistry.java:161) ~[ModelLoaderRegistry.class:?]
at net.minecraftforge.client.model.ModelLoader.func_177590_d(ModelLoader.java:296) ~[ModelLoader.class:?]
at net.minecraft.client.renderer.block.model.ModelBakery.func_177577_b(ModelBakery.java:175) ~[cgb.class:?]
at net.minecraftforge.client.model.ModelLoader.func_177570_a(ModelLoader.java:151) ~[ModelLoader.class:?]
at net.minecraft.client.renderer.block.model.ModelManager.func_110549_a(ModelManager.java:28) [cgc.class:?]
at net.minecraft.client.resources.SimpleReloadableResourceManager.func_110542_a(SimpleReloadableResourceManager.java:121) [cev.class:?]
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:513) [bib.class:?]
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:378) [bib.class:?]
at net.minecraft.client.main.Main.main(SourceFile:123) [Main.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_211]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_211]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_211]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_211]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_211]
at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:196) [NewLaunch.jar:?]
at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:231) [NewLaunch.jar:?]
at org.multimc.EntryPoint.listen(EntryPoint.java:143) [NewLaunch.jar:?]
at org.multimc.EntryPoint.main(EntryPoint.java:34) [NewLaunch.jar:?]
Caused by: net.minecraft.client.renderer.block.model.ModelBlockDefinition$MissingVariantException
at net.minecraft.client.renderer.block.model.ModelBlockDefinition.func_188004_c(ModelBlockDefinition.java:83) ~[bvv.class:?]
at net.minecraftforge.client.model.ModelLoader$VariantLoader.loadModel(ModelLoader.java:1175) ~[ModelLoader$VariantLoader.class:?]
at net.minecraftforge.client.model.ModelLoaderRegistry.getModel(ModelLoaderRegistry.java:157) ~[ModelLoaderRegistry.class:?]
... 22 more

This error goes away when I remove the Tech Reborn mod from the pack.

Forge version: 14.23.5.2838
TR version: 2.22.1.979
Reborn core: 3.13.12.447

Got the same issue, think its related to JEI since that's the only similar mod in my pack (only has tinker's, myst, and heartwell plus optifine). Plus the following errors pop up in the correlation:

[00:36:37] [Client thread/INFO] [minecraft/TextureMap]: Created: 4096x2048 textures-atlas
[00:36:37] [Client thread/INFO] [Config]: [OptiFine] Animated sprites: 123
... bunch of texture and blockstate issues ...
[00:36:39] [Client thread/ERROR] [FML]: Exception loading model for variant minecraft:air#inventory, normal location exception:  ...
[00:36:40] [Client thread/INFO] [jei]: Restarting JEI.
[00:36:40] [Client thread/INFO] [jei]: Starting JEI...

Mind you I'm on 1.12.2 (Java 1.8) with Forge 14.23.5.2860 and LWJGL 2.9.4-nightly-20150209. Plus I do have BetterAdvancements and JER so not sure if their related but hope this helps narrow down the issue for anyone else coming from google. Though IMHO I doubt its anything worth fixing for an older version of nearly everything at this point.

What are the other mods in the pack?