I keep getting an error when i was the Topblock Code.

this.topBlock = Blocks.netherrack;

I try that i didn't work it keeps crashing my game.

Ok

---- Minecraft Crash Report ----
// Ooh. Shiny.

Time: 2/15/15 1:33 PM
Description: Initializing game

java.lang.ClassCastException: net.minecraft.block.BlockNetherrack cannot be cast to net.minecraft.block.state.IBlockState
at com.gameplayer.betterminecraft.worldgen.biome.BiomeHellPlains.<init>(BiomeHellPlains.java:23)
at com.gameplayer.betterminecraft.worldgen.biome.BiomesBM.registerBiomes(BiomesBM.java:27)
at com.gameplayer.betterminecraft.worldgen.biome.BiomesBM.init(BiomesBM.java:22)
at com.gameplayer.betterminecraft.BetterMinecraft.preInit(BetterMinecraft.java:34)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:515)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
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:208)
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:187)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
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:118)
at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:514)
at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:243)
at net.minecraft.client.Minecraft.startGame(Minecraft.java:412)
at net.minecraft.client.Minecraft.run(Minecraft.java:325)
at net.minecraft.client.main.Main.main(Main.java:117)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
at net.minecraftforge.gradle.GradleStartCommon.launch(GradleStartCommon.java:78)
at GradleStart.main(GradleStart.java:45)


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

-- Head --
Stacktrace:
at com.gameplayer.betterminecraft.worldgen.biome.BiomeHellPlains.<init>(BiomeHellPlains.java:23)
at com.gameplayer.betterminecraft.worldgen.biome.BiomesBM.registerBiomes(BiomesBM.java:27)
at com.gameplayer.betterminecraft.worldgen.biome.BiomesBM.init(BiomesBM.java:22)
at com.gameplayer.betterminecraft.BetterMinecraft.preInit(BetterMinecraft.java:34)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:515)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
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:208)
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:187)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
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:118)
at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:514)
at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:243)
at net.minecraft.client.Minecraft.startGame(Minecraft.java:412)

-- Initialization --
Details:
Stacktrace:
at net.minecraft.client.Minecraft.run(Minecraft.java:325)
at net.minecraft.client.main.Main.main(Main.java:117)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
at net.minecraftforge.gradle.GradleStartCommon.launch(GradleStartCommon.java:78)
at GradleStart.main(GradleStart.java:45)

-- System Details --
Details:
Minecraft Version: 1.8
Operating System: Windows 8.1 (x86) version 6.3
Java Version: 1.8.0_20, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 797301648 bytes (760 MB) / 1046937600 bytes (998 MB) up to 1046937600 bytes (998 MB)
JVM Flags: 3 total; -Xincgc -Xmx1024M -Xms1024M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v9.10 FML v8.0.26.1309 Minecraft Forge 11.14.1.1309 4 mods loaded, 4 mods active
mcp{9.05} [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed->Pre-initialized
FML{8.0.26.1309} [Forge Mod Loader] (forgeSrc-1.8-11.14.1.1309.jar) Unloaded->Constructed->Pre-initialized
Forge{11.14.1.1309} [Minecraft Forge] (forgeSrc-1.8-11.14.1.1309.jar) Unloaded->Constructed->Pre-initialized
bm{1.0} [better Minecraft] (bin) Unloaded->Constructed->Errored
Launched Version: 1.8
LWJGL: 2.9.1
OpenGL: AMD Radeon HD 5700 Series GL version 4.4.13283 Compatibility Profile Context 14.501.1003.0, ATI Technologies Inc.
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

Using VBOs: No
Is Modded: Definitely; Client brand changed to 'fml,forge'
Type: Client (map_client.txt)
Resource Packs: []
Current Language: English (US)
Profiler Position: N/A (disabled)

this.topBlock = CMContent.cobaltgrass.getDefaultState();

You did not as I told you. You cannot simply cast a Block to IBlockState.

fixed the problem stupid me thanks diesieben07