It is possible to change the missing block model

It is possible to change the "missing block" model using a resource pack.
If you break the model, then the game has no model to call when it has a broken or missing model, causing the game to crash.

The fix is simple, do what was done with the missing texture, and don't allow it to be changed by resource packs (hard code the model).

I know this could be qualified as "don't do things that obviously crash the game", but this is still a bug.