Crash: Ticking entity 'java.lang.NullPointerException: Ticking entity'

Normal play in survival, and suddenly stops.

java.lang.NullPointerException: Ticking entity
	at ly.y(SourceFile:972)
	at sr$5.a(SourceFile:44)
	at sr$5.apply(SourceFile:41)
	at ajs.a(SourceFile:2524)
	at ajs.a(SourceFile:2514)
	at ajs.a(SourceFile:2503)
	at sx.L(SourceFile:610)
	at sx.cs(SourceFile:634)
	at sw.n(SourceFile:1841)
	at sx.n(SourceFile:499)
	at aaa.n(SourceFile:41)
	at zr.n(SourceFile:233)
	at sw.A_(SourceFile:1703)
	at sx.A_(SourceFile:282)
	at aaa.A_(SourceFile:46)
	at ajs.a(SourceFile:1517)
	at lw.a(SourceFile:718)
	at ajs.h(SourceFile:1495)
	at ajs.k(SourceFile:1378)
	at lw.k(SourceFile:551)
	at net.minecraft.server.MinecraftServer.D(SourceFile:634)
	at lh.D(SourceFile:335)
	at net.minecraft.server.MinecraftServer.C(SourceFile:562)
	at net.minecraft.server.MinecraftServer.run(SourceFile:466)
	at java.lang.Thread.run(Thread.java:745)