Happened a build or 2 ago once... and then again yesterday, not super frequently but does need dealt with, all I could find out about this was as an old problem that should have gone away long ago...

[04:36:11] [Server thread/ERROR]: Encountered an unexpected exception[04:36:11] [Server thread/ERROR]: Encountered an unexpected exceptionnet.minecraft.server.v1_12_R1.ReportedException: Exception ticking world at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:812) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:406) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577) [spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_151]Caused by: java.lang.NullPointerException at org.bukkit.craftbukkit.libs.it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap$MapIterator.nextEntry(Long2ObjectOpenHashMap.java:636) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at org.bukkit.craftbukkit.libs.it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap$ValueIterator.next(Long2ObjectOpenHashMap.java:876) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at org.bukkit.craftbukkit.v1_12_R1.CraftWorld.processChunkGC(CraftWorld.java:1556) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.WorldServer.doTick(WorldServer.java:334) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:801) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] ... 4 more[04:36:11] [Server thread/ERROR]: Cause of unexpected exception wasjava.lang.NullPointerException: null at org.bukkit.craftbukkit.libs.it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap$MapIterator.nextEntry(Long2ObjectOpenHashMap.java:636) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at org.bukkit.craftbukkit.libs.it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap$ValueIterator.next(Long2ObjectOpenHashMap.java:876) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at org.bukkit.craftbukkit.v1_12_R1.CraftWorld.processChunkGC(CraftWorld.java:1556) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.WorldServer.doTick(WorldServer.java:334) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:801) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:406) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679) ~[spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577) [spigot-1.12.2-R0.1-SNAPSHOT.jar:git-Spigot-c3093ef-3fed09c] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_151][04:36:11] [Server thread/ERROR]: This crash report has been saved to: /home/richard/Spigot/Spigot-Server/target/./crash-reports/crash-2018-03-24_04.36.11-server.txt[04:36:11] [Server thread/INFO]: Stopping server