I had a similar issue with physical material masks: once the material was setup with it, when I started play the editor crashed. To fix it I reimported the .png mask, making sure it had ONLY red and green colors (or as many as you need, but starting from top to bottom of the list in the material editor). Infact, the first .png I imported had white and black only pixels, and the PMM importer converted it to a red-black one, which I suppose it had problems reading to