[MC-116656] Closing world during ender dragon respawn and reopening it causes crash

The bug

Closing a world during the ender dragon respawn sequence and reopening it causes a crash. This could be caused by the new advancement "The End... Again..." (end/respawn_dragon.json) in case it tries to track which player placed the end crystals.

Warning: Only try to reproduce this in a seperate test world since the world will be unusable afterwards.

crash-2017-04-26_17.14.52-server.txt

Description: Ticking player

java.lang.NullPointerException: Ticking player
	at bau.a(SourceFile:188)
	at bau.b(SourceFile:135)
	at ayf.a(SourceFile:381)
	at nr.a(SourceFile:227)
	at am$b.a(SourceFile:118)
	at am$a.a(SourceFile:148)
	at am.a(SourceFile:85)
	at nu.k_(SourceFile:390)
	at oe.G_(SourceFile:182)
	at gf.a(SourceFile:231)
	at od.c(SourceFile:187)
	at net.minecraft.server.MinecraftServer.D(SourceFile:652)
	at net.minecraft.server.MinecraftServer.C(SourceFile:562)
	at cgb.C(SourceFile:154)
	at net.minecraft.server.MinecraftServer.run(SourceFile:466)
	at java.lang.Thread.run(Unknown Source)