What do you guys think can help me solve my issue?

---- Minecraft Crash Report ----

WARNING: coremods are present:
SpongeCoremod (spongeforge-1.8.9-1808-4.1.0-BETA-1244.jar)
Java8CheckerCoremod (Pixelmon-1.8.9-4.2.3-universal.jar)
Contact their authors BEFORE contacting forge

// Oh - I know what I did wrong!

Time: 4/7/16 2:24 AM
Description: Exception ticking world

java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.remove(Unknown Source)
at org.spongepowered.mod.event.SpongeForgeEventFactory.callEntityJoinWorldEvent(SpongeForgeEventFactory.java:873)
at org.spongepowered.mod.event.SpongeForgeEventFactory.callForgeEvent(SpongeForgeEventFactory.java:341)
at org.spongepowered.mod.event.SpongeModEventManager.postBulk(SpongeModEventManager.java:221)
at org.spongepowered.mod.event.SpongeModEventManager.post(SpongeModEventManager.java:265)
at org.spongepowered.mod.event.SpongeModEventManager.post(SpongeModEventManager.java:251)
at org.spongepowered.common.SpongeImpl.postEvent(SpongeImpl.java:128)
at org.spongepowered.common.event.CauseTracker.handleEntitySpawns(CauseTracker.java:368)
at org.spongepowered.common.event.CauseTracker.handlePostTickCaptures(CauseTracker.java:515)
at org.spongepowered.common.event.CauseTracker.randomTickBlock(CauseTracker.java:1076)
at net.minecraft.world.WorldServer.redirect$onUpdateBlocks$0(WorldServer.java:120)
at net.minecraft.world.WorldServer.func_147456_g(WorldServer.java:431)
at net.minecraft.world.WorldServer.func_72835_b(WorldServer.java:221)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:690)
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:344)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:605)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:481)
at java.lang.Thread.run(Unknown Source)

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

– Head –
Stacktrace:
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.remove(Unknown Source)
at org.spongepowered.mod.event.SpongeForgeEventFactory.callEntityJoinWorldEvent(SpongeForgeEventFactory.java:873)
at org.spongepowered.mod.event.SpongeForgeEventFactory.callForgeEvent(SpongeForgeEventFactory.java:341)
at org.spongepowered.mod.event.SpongeModEventManager.postBulk(SpongeModEventManager.java:221)
at org.spongepowered.mod.event.SpongeModEventManager.post(SpongeModEventManager.java:265)
at org.spongepowered.mod.event.SpongeModEventManager.post(SpongeModEventManager.java:251)
at org.spongepowered.common.SpongeImpl.postEvent(SpongeImpl.java:128)
at org.spongepowered.common.event.CauseTracker.handleEntitySpawns(CauseTracker.java:368)
at org.spongepowered.common.event.CauseTracker.handlePostTickCaptures(CauseTracker.java:515)
at org.spongepowered.common.event.CauseTracker.randomTickBlock(CauseTracker.java:1076)
at net.minecraft.world.WorldServer.redirect$onUpdateBlocks$0(WorldServer.java:120)
at net.minecraft.world.WorldServer.func_147456_g(WorldServer.java:431)
at net.minecraft.world.WorldServer.func_72835_b(WorldServer.java:221)

– Affected level –
Details:
Level name: WorldHub
All players: 1 total; [EntityPlayerMP[‘EchoTV’/50, l=‘WorldHub’, x=-22.86, y=74.59, z=-87.43]]
Chunk stats: ServerChunkCache: 838 Drop: 0
Level seed: -5978826861160794796
Level generator: ID 01 - flat, ver 0. Features enabled: true
Level generator options:
Level spawn location: -95.00,4.00,-204.00 - World: (-95,4,-204), Chunk: (at 1,0,4 in -6,-13; contains blocks -96,0,-208 to -81,255,-193), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
Level time: 76218 game time, 58174 day time
Level dimension: 4
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 1 (now: false), thunder time: 1 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:690)
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:344)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:605)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:481)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 1.8.9
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_77, Oracle Corporation
Java VM Version: Java HotSpot™ 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 607696928 bytes (579 MB) / 1766850560 bytes (1685 MB) up to 1901592576 bytes (1813 MB)
JVM Flags: 0 total;
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP 9.19 Powered by Forge 11.15.1.1808 7 mods loaded, 7 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.jar)
UCHIJAAAA Forge{11.15.1.1808} [Minecraft Forge] (forge.jar)
UCHIJAAAA sponge{DEV} [SpongeForge] (spongeforge-1.8.9-1808-4.1.0-BETA-1244.jar)
UCHIJAAAA io.github.hsyyid.essentialcmds{8.0.1-27167e0} [EssentialCmds] (EssentialCmds-8.0.1.jar)
UCHIJAAAA pixelmon{4.2.3} [Pixelmon] (Pixelmon-1.8.9-4.2.3-universal.jar)
UCHIJAAAA worldedit{6.1.1} [WorldEdit] (worldedit-forge-mc1.8.9-6.1.1.jar)
Loaded coremods (and transformers):
SpongeCoremod (spongeforge-1.8.9-1808-4.1.0-BETA-1244.jar)
org.spongepowered.common.launch.transformer.SpongeSuperclassTransformer
Java8CheckerCoremod (Pixelmon-1.8.9-4.2.3-universal.jar)

Profiler Position: N/A (disabled)
Player Count: 1 / 20; [EntityPlayerMP['EchoTV'/50, l='WorldHub', x=-22.86, y=74.59, z=-87.43]]
Is Modded: Definitely; Server brand changed to 'fml,forge,sponge'
Type: Dedicated Server (map_server.txt)

Looks like Pixelmon is changing things from the wrong thread, contact their devs with this trace.