[MC-181833] Crash on world startup after playing world in 20w18a

The Bug

After playing on my test world in 20w18a, exiting it, and joining again, it now crashes on startup.
My worldfile: https://www.dropbox.com/s/zxry677t6t4zlv3/Testwelt.zip?dl=0
I also noticed nullpointer exceptions when trying to save chunks in the launcher log: 2020-05-01-2.log
---- Minecraft Crash Report ----
// Shall we play a game?

Time: 01.05.20 00:20
Description: Ticking memory connection

java.lang.NullPointerException: Ticking memory connection
	at ms$a$$Lambda$3061/55543660.apply(Unknown Source)
	at ms$a.a(SourceFile:346)
	at ms.b(SourceFile:108)
	at my$a.a(SourceFile:460)
	at my$a.serialize(SourceFile:330)
	at com.google.gson.internal.bind.TreeTypeAdapter.write(TreeTypeAdapter.java:81)
	at com.google.gson.Gson.toJson(Gson.java:669)
	at com.google.gson.Gson.toJsonTree(Gson.java:562)
	at com.google.gson.Gson.toJsonTree(Gson.java:541)
	at com.google.gson.internal.bind.TreeTypeAdapter$GsonContextImpl.serialize(TreeTypeAdapter.java:155)
	at mp$b.a(SourceFile:260)
	at mp$b.a(SourceFile:275)
	at mp$b.a(SourceFile:282)
	at mp$b.a(SourceFile:282)
	at mp$b.serialize(SourceFile:132)
	at com.google.gson.internal.bind.TreeTypeAdapter.write(TreeTypeAdapter.java:81)
	at com.google.gson.Gson.toJson(Gson.java:669)
	at com.google.gson.Gson.toJson(Gson.java:648)
	at com.google.gson.Gson.toJson(Gson.java:603)
	at com.google.gson.Gson.toJson(Gson.java:583)
	at mp$b.a(SourceFile:343)
	at cdh.a(SourceFile:46)
	at cdh.b(SourceFile:102)
	at or.<init>(SourceFile:71)
	at yf.a(SourceFile:1096)
	at yf.a(SourceFile:780)
	at yf.a(SourceFile:903)
	at yf.a(SourceFile:1029)
	at yr.c(SourceFile:472)
	at yt.m(SourceFile:870)
	at yt.f(SourceFile:785)
	at yt.c(SourceFile:764)
	at abe.a(SourceFile:192)
	at zn.c(SourceFile:111)
	at zn.b(SourceFile:63)
	at mc.a(SourceFile:230)
	at zk.c(SourceFile:171)
	at net.minecraft.server.MinecraftServer.b(SourceFile:912)
	at net.minecraft.server.MinecraftServer.a(SourceFile:839)
	at enx.a(SourceFile:89)
	at net.minecraft.server.MinecraftServer.run(SourceFile:698)
	at java.lang.Thread.run(Thread.java:745)


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

-- Head --
Thread: Server thread
Stacktrace:
	at ms$a$$Lambda$3061/55543660.apply(Unknown Source)
	at ms$a.a(SourceFile:346)
	at ms.b(SourceFile:108)
	at my$a.a(SourceFile:460)
	at my$a.serialize(SourceFile:330)
	at com.google.gson.internal.bind.TreeTypeAdapter.write(TreeTypeAdapter.java:81)
	at com.google.gson.Gson.toJson(Gson.java:669)
	at com.google.gson.Gson.toJsonTree(Gson.java:562)
	at com.google.gson.Gson.toJsonTree(Gson.java:541)
	at com.google.gson.internal.bind.TreeTypeAdapter$GsonContextImpl.serialize(TreeTypeAdapter.java:155)
	at mp$b.a(SourceFile:260)
	at mp$b.a(SourceFile:275)
	at mp$b.a(SourceFile:282)
	at mp$b.a(SourceFile:282)
	at mp$b.serialize(SourceFile:132)
	at com.google.gson.internal.bind.TreeTypeAdapter.write(TreeTypeAdapter.java:81)
	at com.google.gson.Gson.toJson(Gson.java:669)
	at com.google.gson.Gson.toJson(Gson.java:648)
	at com.google.gson.Gson.toJson(Gson.java:603)
	at com.google.gson.Gson.toJson(Gson.java:583)
	at mp$b.a(SourceFile:343)
	at cdh.a(SourceFile:46)
	at cdh.b(SourceFile:102)
	at or.<init>(SourceFile:71)
	at yf.a(SourceFile:1096)
	at yf.a(SourceFile:780)
	at yf.a(SourceFile:903)
	at yf.a(SourceFile:1029)
	at yr.c(SourceFile:472)
	at yt.m(SourceFile:870)
	at yt.f(SourceFile:785)
	at yt.c(SourceFile:764)
	at abe.a(SourceFile:192)
	at zn.c(SourceFile:111)
	at zn.b(SourceFile:63)
	at mc.a(SourceFile:230)

-- Ticking connection --
Details:
	Connection: mc@4c2bae42
Stacktrace:
	at zk.c(SourceFile:171)
	at net.minecraft.server.MinecraftServer.b(SourceFile:912)
	at net.minecraft.server.MinecraftServer.a(SourceFile:839)
	at enx.a(SourceFile:89)
	at net.minecraft.server.MinecraftServer.run(SourceFile:698)
	at java.lang.Thread.run(Thread.java:745)

-- System Details --
Details:
	Minecraft Version: 20w18a
	Minecraft Version ID: 20w18a
	Operating System: Windows 10 (amd64) version 10.0
	Java Version: 1.8.0_51, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 2242910264 bytes (2139 MB) / 3791650816 bytes (3616 MB) up to 4294967296 bytes (4096 MB)
	CPUs: 4
	JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx4096M -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
	Player Count: 1 / 8; [yu['NeunEinser'/4604, l='ServerLevel[Testwelt]', x=-1644.90, y=56.00, z=-945.50]]
	Data Packs: vanilla
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.