Saving a project with a sprite that doesn't have a valid mask will crash the tool

[Grossley]

Saving a project with a sprite that doesn't have a valid mask will crash UTMT iirc

Please verify if this is true

[Grossley]

To reproduce, make a sprite bigger than the mask, making the mask invalid, and then save the game.

[Jacky720]

This currently fails an assert and terminates the save process without crashing. @Grossley please close if this is the desired behavior.