ArrayIndexOutOfBoundsException crash when loading singleplayer world. Launcher said my game is modded and wouldn't send report, it is not.

Double-clicked my singleplayer survival world to start playing. Game crashed. Launcher says "we think your game is modded" and would not accept my crash report, even though in the crash details it says "Is Modded: Probably not." My game is 100% vanilla, no mods, no texture packs.

I double-clicked on my world very quickly after the world screen came up.. maybe it was still doing something and I clicked it too fast.

Crash log attached.