Null pointer exception on player quit

[11:30:29] [Server thread/INFO]: ridddle left the game
q: Saving entity NBT
        at ru.e(SourceFile:1149)
        at ru.d(SourceFile:1116)
        at arn.a(SourceFile:210)
        at arn.a(SourceFile:99)
        at nl.b(SourceFile:160)
        at nl.d(SourceFile:219)
        at nm.c(SourceFile:145)
        at net.minecraft.server.MinecraftServer.v(SourceFile:519)
        at mm.v(SourceFile:264)
        at net.minecraft.server.MinecraftServer.u(SourceFile:461)
        at net.minecraft.server.MinecraftServer.run(SourceFile:377)
        at mc.run(SourceFile:647)
Caused by: java.lang.NullPointerException
        at agx.j(SourceFile:115)
        at agy.a(SourceFile:125)
        at yw.b(SourceFile:212)
        at ru.e(SourceFile:1137)
        ... 11 more
q: Saving entity NBT
        at ru.e(SourceFile:1149)
        at ru.d(SourceFile:1116)
        at arn.a(SourceFile:210)
        at arn.a(SourceFile:99)
        at nl.b(SourceFile:160)
        at nl.a(SourceFile:191)
        at nm.a(SourceFile:655)
        at net.minecraft.server.MinecraftServer.a(SourceFile:295)
        at net.minecraft.server.MinecraftServer.u(SourceFile:479)
        at net.minecraft.server.MinecraftServer.run(SourceFile:377)
        at mc.run(SourceFile:647)
Caused by: java.lang.NullPointerException
        at agx.j(SourceFile:115)
        at agy.a(SourceFile:125)
        at yw.b(SourceFile:212)
        at ru.e(SourceFile:1137)
        ... 10 more
q: Saving entity NBT
        at ru.e(SourceFile:1149)
        at ru.d(SourceFile:1116)
        at arn.a(SourceFile:210)
        at arn.a(SourceFile:99)
        at nl.b(SourceFile:160)
        at nl.d(SourceFile:219)
        at nm.c(SourceFile:145)
        at net.minecraft.server.MinecraftServer.v(SourceFile:519)
        at mm.v(SourceFile:264)
        at net.minecraft.server.MinecraftServer.u(SourceFile:461)
        at net.minecraft.server.MinecraftServer.run(SourceFile:377)
        at mc.run(SourceFile:647)
Caused by: java.lang.NullPointerException
        at agx.j(SourceFile:115)
        at agy.a(SourceFile:125)
        at yw.b(SourceFile:212)
        at ru.e(SourceFile:1137)
        ... 11 more

They just keep coming, repeatedly, even after I /save-all. Reboot helps.