THIS IS NOT A DUPE OF MC-75777. THIS IS DIFFERENT.
for some reason, when i left my world, the output gave me this error. But it didn't crash. it acted normally.
The attached file is the world in which it happened

[12:27:39] [Server thread/INFO]: dropper99 lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[12:27:39] [Server thread/INFO]: dropper99 left the game
[12:27:39] [Server thread/INFO]: Stopping singleplayer server as player logged out
[12:27:50] [Server thread/INFO]: Stopping server
[12:27:50] [Server thread/INFO]: Saving players
[12:27:50] [Server thread/INFO]: Saving worlds
[12:27:50] [Server thread/INFO]: Saving chunks for level 'Construction'/Overworld
[12:27:50] [Server thread/INFO]: Saving chunks for level 'Construction'/Nether
[12:27:50] [Server thread/INFO]: Saving chunks for level 'Construction'/The End
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at jz.a(SourceFile:153) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:355) ~[1.8.2-pre1.jar:?]
at jz.a(SourceFile:144) ~[1.8.2-pre1.jar:?]
at kb.a(SourceFile:21) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at kr.a(SourceFile:42) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:374) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:38) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:11) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at jz.a(SourceFile:153) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:355) ~[1.8.2-pre1.jar:?]
at jz.a(SourceFile:144) ~[1.8.2-pre1.jar:?]
at ka.a(SourceFile:61) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at kr.a(SourceFile:42) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:374) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:38) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:11) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at jz.a(SourceFile:153) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:355) ~[1.8.2-pre1.jar:?]
at jz.a(SourceFile:144) ~[1.8.2-pre1.jar:?]
at kb.a(SourceFile:21) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at kr.a(SourceFile:42) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:374) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:38) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:11) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at cee.a(SourceFile:324) ~[1.8.2-pre1.jar:?]
at lp.a(SourceFile:66) ~[1.8.2-pre1.jar:?]
at lp.a(SourceFile:10) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at kr.a(SourceFile:42) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:374) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:38) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:11) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at jz.a(SourceFile:153) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:355) ~[1.8.2-pre1.jar:?]
at jz.a(SourceFile:144) ~[1.8.2-pre1.jar:?]
at kb.a(SourceFile:21) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more
[12:27:54] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:45) [1.8.2-pre1.jar:?]
at bst.at(SourceFile:876) [1.8.2-pre1.jar:?]
at bst.a(SourceFile:321) [1.8.2-pre1.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.2-pre1.jar:?]
Caused by: java.lang.NullPointerException
at kr.a(SourceFile:42) ~[1.8.2-pre1.jar:?]
at cee.a(SourceFile:374) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:38) ~[1.8.2-pre1.jar:?]
at kr.a(SourceFile:11) ~[1.8.2-pre1.jar:?]
at ii.run(SourceFile:13) ~[1.8.2-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.7.0_51]
at w.a(SourceFile:44) ~[1.8.2-pre1.jar:?]
... 3 more