The Bug:

Reloading or changing your resource pack will cause a crash after you change a block model. I managed to reproduce this many times after changing the petrified oak slab models.

To Reproduce:

1. Create a new world, or log into an existing one and put on a resource pack
2. Change a block model .json file in the resource pack you have on
3. Reload resource packs
   → The game crashes and fails to write a crash report

Another Option: (assuming the first steps didn't work)

1. Do the first steps
2. Put on then remove another resource pack
3. → The game crashes and fails to write a crash report