[MC-137457] Crash upon opening the F3 menu

The bug

The game might crash when you open the F3 menu.
Tweet from [Mojang] Nathan Adams

... and it looks like we need a 'b' already If you close and reopen the F3 overlay you might crash. Don't do that

Crash report

18w43a: crash-2018-10-24_14.09.02-client.txt

Description: Unexpected error

java.lang.NullPointerException: Unexpected error
	at ckm.e(SourceFile:213)
	at ckm.c(SourceFile:94)
	at ckm.b(SourceFile:81)
	at cka.a(SourceFile:250)
	at cxp.a(SourceFile:779)
	at cjj.c(SourceFile:817)
	at cjj.a(SourceFile:380)
	at net.minecraft.client.main.Main.main(SourceFile:144)