I was working on updating a 1.12.2 resource pack to work in 18w21b while still working in 1.12.2. When I was working on the models for the pistons, every texture in the game suddenly became the "Missing texture" texture. I tried restarting 18w21a several times, but whenever I loaded the resource pack, it removed itself from the Selected Resource Packs list and all the textures became the Missing texture. When I tried the same resource pack in 1.12.2, it removed itself from the Selected Resource Packs list but the default textures stayed. Attached should be the resource pack which caused this to happen.