When i stopped my server this was printed into the console (only for overworld):

[14:30:42] [Server thread/INFO]: Saving chunks for level 'world'/overworld
[14:30:42] [Server thread/INFO]: [STDERR]: java.lang.NullPointerException
[14:30:42] [Server thread/INFO]: [STDERR]: at bzz.a(SourceFile:348)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.HashMap$Values.forEach(HashMap.java:980)
[14:30:42] [Server thread/INFO]: [STDERR]: at bzz.a(SourceFile:345)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.HashMap$ValueSpliterator.forEachRemaining(HashMap.java:1620)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
[14:30:42] [Server thread/INFO]: [STDERR]: at bzz.i(SourceFile:344)
[14:30:42] [Server thread/INFO]: [STDERR]: at caa.b(SourceFile:146)
[14:30:42] [Server thread/INFO]: [STDERR]: at bxi.a(SourceFile:89)
[14:30:42] [Server thread/INFO]: [STDERR]: at bxi.a(SourceFile:75)
[14:30:42] [Server thread/INFO]: [STDERR]: at sh.a(SourceFile:889)
[14:30:42] [Server thread/INFO]: [STDERR]: at sh.a(SourceFile:840)
[14:30:42] [Server thread/INFO]: [STDERR]: at net.minecraft.server.MinecraftServer.a(SourceFile:446)
[14:30:42] [Server thread/INFO]: [STDERR]: at net.minecraft.server.MinecraftServer.j_(SourceFile:471)
[14:30:42] [Server thread/INFO]: [STDERR]: at net.minecraft.server.MinecraftServer.run(SourceFile:565)
[14:30:42] [Server thread/INFO]: [STDERR]: at java.lang.Thread.run(Thread.java:745)