Summary:
If you use the tag command to tag the ender dragon and exit the world, it causes the world to crash whenever you reenter that world, making it completely unplayable.

Steps to Reproduce:

1. Create a new world
2. Go to the End
3. /tag @e[type=ender_dragon] add (anything_here)
4. Exit and reload the world

Observed Results:
Game crashes.

Expected Results:
Game should not crash.