Constant loss of

[westside47]

I constantly get this error show up saying the spawn rule the mask was using is missing in the scene, even though nothing with the rules has changed. Is there a bug or way to stop this from breaking the association?

[Bryan]

This usually happens when one of the textures has changed on the terrain. 
The best thing to do (if you have not manually painted) is to use the resource helper and remove the terrain textures. 
Then spawn the textures based off the biome, from there assign the correct texture.