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.
Description: Ticking player
java.lang.NullPointerException: Ticking player
at java.lang.Thread.run(Unknown Source)