Steps to reproduce

1. Destroy the End Crystals
2. Build a pillar directly on top of the exit portal (x=0, z=0) until you reach y=128 (the coordinates of the topmost block should be x=0, y=128, z=0; higher pillars should work as well)
3. Stand on top of the pillar, and wait for the Ender Dragon to reach you
4. A couple of seconds after the Dragon stops above the pillar, the game should crash.
   This doesn't always happen, and it may require creating a different world if it doesn't work on your first attempt.

By analyzing the log from a Forge instance, I was able to find out the bug is related to the Ender Dragon pathfinding code. After the server (or client if in singleplayer) crashes the first time, it will crash again everytime the End is loaded, unless you delete the DIM1 folder.