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Today I downloaded Pixelmon 4.2.1, and when i tried to go into a single player world, minecraft became unresponsive after it stopped saying "building terrain."

I also got this crash report message:

---- Minecraft Crash Report ----

WARNING: coremods are present:
Java8CheckerCoremod (Pixelmon-1.8.9-4.2.1-universal.jar)
Contact their authors BEFORE contacting forge

// I blame Dinnerbone.

Time: 2/23/16 7:54 PM
Description: Exception in server tick loop

java.lang.NoClassDefFoundError: com/pixelmonmod/pixelmon/worldGeneration/structure/util/GeneralScattered$1
at com.pixelmonmod.pixelmon.worldGeneration.structure.util.GeneralScattered.func_74865_a(GeneralScattered.java:204)
at com.pixelmonmod.pixelmon.worldGeneration.structure.util.GeneralScattered.func_74875_a(GeneralScattered.java:90)
at com.pixelmonmod.pixelmon.worldGeneration.structure.towns.ComponentTownPart.func_74875_a(ComponentTownPart.java:109)
at net.minecraft.world.gen.structure.StructureStart.func_75068_a(StructureStart.java:46)
at net.minecraft.world.gen.structure.MapGenStructure.func_175794_a(MapGenStructure.java:86)
at net.minecraft.world.gen.ChunkProviderGenerate.func_73153_a(ChunkProviderGenerate.java:401)
at net.minecraft.world.gen.ChunkProviderServer.func_73153_a(ChunkProviderServer.java:261)
at net.minecraft.world.chunk.Chunk.func_76624_a(Chunk.java:1039)
at net.minecraft.world.gen.ChunkProviderServer.originalLoadChunk(ChunkProviderServer.java:172)
at net.minecraft.world.gen.ChunkProviderServer.loadChunk(ChunkProviderServer.java:114)
at net.minecraft.server.management.PlayerManager$PlayerInstance.<init>(PlayerManager.java:360)
at net.minecraft.server.management.PlayerManager.func_72690_a(PlayerManager.java:94)
at net.minecraft.server.management.PlayerManager.func_72683_a(PlayerManager.java:135)
at net.minecraft.server.management.ServerConfigurationManager.func_72375_a(ServerConfigurationManager.java:258)
at net.minecraft.server.management.ServerConfigurationManager.func_72377_c(ServerConfigurationManager.java:321)
at net.minecraft.server.management.ServerConfigurationManager.initializeConnectionToPlayer(ServerConfigurationManager.java:159)
at net.minecraftforge.fml.common.network.handshake.NetworkDispatcher.completeServerSideConnection(NetworkDispatcher.java:239)
at net.minecraftforge.fml.common.network.handshake.NetworkDispatcher.access$100(NetworkDispatcher.java:54)
at net.minecraftforge.fml.common.network.handshake.NetworkDispatcher$1.func_73660_a(NetworkDispatcher.java:190)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:289)
at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:189)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:724)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:605)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:152)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:481)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: com.pixelmonmod.pixelmon.worldGeneration.structure.util.GeneralScattered$1
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)
... 26 more
Caused by: java.lang.OutOfMemoryError: Java heap space
at java.util.jar.Attributes.<init>(Attributes.java:84)
at java.util.jar.Attributes.clone(Attributes.java:291)
at sun.net.www.protocol.jar.URLJarFile.getManifest(URLJarFile.java:155)
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:148)
... 28 more

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

-- System Details --
Details:
Minecraft Version: 1.8.9
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 35538376 bytes (33 MB) / 1060372480 bytes (1011 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 1, tcache: 1, allocated: 12, tallocated: 94
FML: MCP 9.19 Powered by Forge 11.15.1.1722 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
UCHIJAAAA mcp{9.19} [Minecraft Coder Pack] (minecraft.jar)
UCHIJAAAA FML{8.0.99.99} [Forge Mod Loader] (forge-1.8.9-11.15.1.1722.jar)
UCHIJAAAA Forge{11.15.1.1722} [Minecraft Forge] (forge-1.8.9-11.15.1.1722.jar)
UCHIJAAAA pixelmon{4.2.1} [Pixelmon] (Pixelmon-1.8.9-4.2.1-universal.jar)
Loaded coremods (and transformers):
Java8CheckerCoremod (Pixelmon-1.8.9-4.2.1-universal.jar)

GL info: ~~ERROR~~ RuntimeException: No OpenGL context found in the current thread.
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [EntityPlayerMP['xBlackDeath'/467, l='New World-', x=-24.50, y=74.00, z=233.50]]
Type: Integrated Server (map_client.txt)
Is Modded: Definitely; Client brand changed to 'fml,forge'

I do not know what to do, not being a Minecraft, nor technology in general expert.

HELP ME!

Thanks.