Server crashed with "java.lang.NullPointerException: Cannot read the array length because "<local2>" is null"

The server was running normally when it suddenly crashed with:

java.lang.NullPointerException: Cannot read the array length because "<local2>" is null
    at java.base/java.lang.StringLatin1.hashCode(StringLatin1.java:196)
    at java.base/java.lang.String.hashCode(String.java:2339)
    at it.unimi.dsi.fastutil.objects.Object2ObjectOpenHashMap.get(Object2ObjectOpenHashMap.java:322)
    at emx.e(SourceFile:324)
    at blv.cg(SourceFile:2482)
    at bly.a(SourceFile:50)
    at bml.M_(SourceFile:2825)
    at bml.d_(SourceFile:2771)
    at bmn.d_(SourceFile:556)
    at blq.d_(SourceFile:128)
    at bxy.d_(SourceFile:58)
    at byu.d_(SourceFile:142)
    at bml.l(SourceFile:2446)
    at bmn.l(SourceFile:357)
    at and.a(SourceFile:761)
    at ctp.a(SourceFile:492)
    at and.a(SourceFile:399)
    at dne.a(SourceFile:54)
    at and.a(SourceFile:363)
    at net.minecraft.server.MinecraftServer.b(SourceFile:948)
    at ama.b(SourceFile:283)
    at net.minecraft.server.MinecraftServer.a(SourceFile:845)
    at net.minecraft.server.MinecraftServer.w(SourceFile:683)
    at net.minecraft.server.MinecraftServer.a(SourceFile:270)
    at java.base/java.lang.Thread.run(Thread.java:833)

The rest of the crash report points towards a sheep as the source of the crash.