---- Minecraft Crash Report ----
// My bad.
Time: 21.05.18 09:06
Description: Exception in server tick loop

java.lang.IllegalStateException: Cannot build unresolved tag entry
at wg$c.a(SourceFile:228) 
at wg.<init>(SourceFile:41) 
at wg$a.b(SourceFile:134) 
at wh.a(SourceFile:137) 
at wi.a(SourceFile:36) 
at uo.c(SourceFile:123) 
at uo.a(SourceFile:106) 
at net.minecraft.server.MinecraftServer.a(SourceFile:1370) 
at net.minecraft.server.MinecraftServer.a(SourceFile:356) 
at net.minecraft.server.MinecraftServer.a(SourceFile:299) 
at rv.d(SourceFile:248) at net.minecraft.server.MinecraftServer.run(SourceFile:502) 
at java.lang.Thread.run(Unknown Source)

A detailed walkthrough of the error, its code path and all known details is as follows:
----------------------------------------------------------------------------------
-- System Details --
Details: Minecraft Version: 18w20c 
Operating System: Windows 8.1 (amd64) version 6.3 
Java Version: 1.8.0_171, Oracle Corporation 
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation 
Memory: 859455328 bytes (819 MB) / 1029177344 bytes (981 MB) up to 1029177344 bytes (981 MB) 
JVM Flags: 2 total; -Xmx1024M -Xms1024M 
Profiler Position: N/A (disabled) 
Player Count: 0 / 12; [] 
Data Packs: vanilla, file/cod (incompatible), file/guns (incompatible) 
Is Modded: Unknown (can't tell) 
Type: Dedicated Server (map_server.txt)