Due to the TPS issue in pre5 we decided to roll back to pre4, however that cause all sorts of issues, mainly the NullPointerException and causing the server to be unable to save chunks, presumably due to changes done in pre5.
[22:55:35] [Server thread/ERROR]: Failed to save chunk
java.lang.NullPointerException: null
at cay.a(SourceFile:115) ~[minecraft_server.1.13-pre4.jar:?]
at brq.a(SourceFile:693) ~[minecraft_server.1.13-pre4.jar:?]
at brq.a(SourceFile:455) ~[minecraft_server.1.13-pre4.jar:?]
at brq.a(SourceFile:215) ~[minecraft_server.1.13-pre4.jar:?]
at sw.b(SourceFile:217) ~[minecraft_server.1.13-pre4.jar:?]
at sw.a(SourceFile:237) ~[minecraft_server.1.13-pre4.jar:?]
at sx.a(SourceFile:730) ~[minecraft_server.1.13-pre4.jar:?]
at rd.a(SourceFile:58) ~[minecraft_server.1.13-pre4.jar:?]
at rd.a(SourceFile:39) ~[minecraft_server.1.13-pre4.jar:?]
at rd.b(SourceFile:22) ~[minecraft_server.1.13-pre4.jar:?]
at com.mojang.brigadier.CommandDispatcher.execute(CommandDispatcher.java:128) ~[minecraft_server.1.13-pre4.jar:?]
at com.mojang.brigadier.CommandDispatcher.execute(CommandDispatcher.java:68) ~[minecraft_server.1.13-pre4.jar:?]
at bv.a(SourceFile:202) ~[minecraft_server.1.13-pre4.jar:?]
at tw.c(SourceFile:1035) ~[minecraft_server.1.13-pre4.jar:?]
at tw.a(SourceFile:1022) ~[minecraft_server.1.13-pre4.jar:?]
at md.a(SourceFile:37) ~[minecraft_server.1.13-pre4.jar:?]
at md.a(SourceFile:9) ~[minecraft_server.1.13-pre4.jar:?]
at iv.a(SourceFile:10) ~[minecraft_server.1.13-pre4.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_171]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_171]
at k.a(SourceFile:176) [minecraft_server.1.13-pre4.jar:?]
at net.minecraft.server.MinecraftServer.w(SourceFile:674) [minecraft_server.1.13-pre4.jar:?]
at si.w(SourceFile:356) [minecraft_server.1.13-pre4.jar:?]
at net.minecraft.server.MinecraftServer.v(SourceFile:630) [minecraft_server.1.13-pre4.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:535) [minecraft_server.1.13-pre4.jar:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_171][22:55:35] [Server thread/ERROR]: Failed to save chunk
java.lang.NullPointerException: null
at cay.a(SourceFile:115) ~[minecraft_server.1.13-pre4.jar:?]
at brq.a(SourceFile:693) ~[minecraft_server.1.13-pre4.jar:?]
at brq.a(SourceFile:455) ~[minecraft_server.1.13-pre4.jar:?]
at brq.a(SourceFile:215) ~[minecraft_server.1.13-pre4.jar:?]
at sw.b(SourceFile:217) ~[minecraft_server.1.13-pre4.jar:?]
at sw.a(SourceFile:237) ~[minecraft_server.1.13-pre4.jar:?]
at sx.a(SourceFile:730) ~[minecraft_server.1.13-pre4.jar:?]
at rd.a(SourceFile:58) ~[minecraft_server.1.13-pre4.jar:?]
at rd.a(SourceFile:39) ~[minecraft_server.1.13-pre4.jar:?]
at rd.b(SourceFile:22) ~[minecraft_server.1.13-pre4.jar:?]
at com.mojang.brigadier.CommandDispatcher.execute(CommandDispatcher.java:128) ~[minecraft_server.1.13-pre4.jar:?]
at com.mojang.brigadier.CommandDispatcher.execute(CommandDispatcher.java:68) ~[minecraft_server.1.13-pre4.jar:?]
at bv.a(SourceFile:202) ~[minecraft_server.1.13-pre4.jar:?]
at tw.c(SourceFile:1035) ~[minecraft_server.1.13-pre4.jar:?]
at tw.a(SourceFile:1022) ~[minecraft_server.1.13-pre4.jar:?]
at md.a(SourceFile:37) ~[minecraft_server.1.13-pre4.jar:?]
at md.a(SourceFile:9) ~[minecraft_server.1.13-pre4.jar:?]
at iv.a(SourceFile:10) ~[minecraft_server.1.13-pre4.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_171]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_171]
at k.a(SourceFile:176) [minecraft_server.1.13-pre4.jar:?]
at net.minecraft.server.MinecraftServer.w(SourceFile:674) [minecraft_server.1.13-pre4.jar:?]
at si.w(SourceFile:356) [minecraft_server.1.13-pre4.jar:?]
at net.minecraft.server.MinecraftServer.v(SourceFile:630) [minecraft_server.1.13-pre4.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:535) [minecraft_server.1.13-pre4.jar:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_171]
Downgrading is not supported.