This error appears when running the Server-side version of the current Github Code:

---- Minecraft Crash Report ----
// Would you like a cupcake?

Time: 11/30/16 3:38 PM
Description: Exception in server tick loop

net.minecraftforge.fml.common.LoaderException: java.lang.NoClassDefFoundError: net/minecraft/client/gui/GuiScreen
at net.minecraftforge.fml.common.LoadController.transition(LoadController.java:163)
at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:553)
at net.minecraftforge.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:88)
at net.minecraftforge.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:355)
at net.minecraft.server.dedicated.DedicatedServer.startServer(DedicatedServer.java:120)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:500)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/gui/GuiScreen
at com.flansmod.common.driveables.DriveableType.read(DriveableType.java:548)
at com.flansmod.common.driveables.VehicleType.read(VehicleType.java:48)
at com.flansmod.common.types.InfoType.read(InfoType.java:74)
at com.flansmod.common.FlansMod.readContentPacks(FlansMod.java:486)
at com.flansmod.common.FlansMod.preInit(FlansMod.java:170)
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:498)
at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:553)
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:498)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:212)
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:190)
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:498)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:119)
at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:550)
... 5 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.gui.GuiScreen
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 37 more
Caused by: net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerException: Exception in class transformer net.minecraftforge.fml.common.asm.transformers.SideTransformer@3bc735b3 from coremod FMLCorePlugin
at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:234)
at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279)
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176)
... 39 more
Caused by: java.lang.RuntimeException: Attempted to load class net/minecraft/client/gui/GuiScreen for invalid side SERVER
at net.minecraftforge.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:49)
at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:230)
... 41 more

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

-- System Details --
Details:
Minecraft Version: 1.8
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_91, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 1104416216 bytes (1053 MB) / 1221066752 bytes (1164 MB) up to 3695181824 bytes (3524 MB)
JVM Flags: 0 total;
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v9.10 FML v8.0.99.99 Minecraft Forge 11.14.4.1577 4 mods loaded, 4 mods active
States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored
UCH mcp{9.05} [Minecraft Coder Pack] (minecraft.jar)
UCH FML{8.0.99.99} [Forge Mod Loader] (forgeSrc-1.8-11.14.4.1577.jar)
UCH Forge{11.14.4.1577} [Minecraft Forge] (forgeSrc-1.8-11.14.4.1577.jar)
UCE flansmod{@Version@} [Flan's Mod] (FlansMod_main)
Loaded coremods (and transformers):
Profiler Position: N/A (disabled)
Is Modded: Definitely; Server brand changed to 'fml,forge'
Type: Dedicated Server (map_server.txt)