Perhaps the mesh wasn't downloaded properly and the texture your trying to add isn't Malfunctioning correctly.
My advise is to check if everything in the mesh folder is put in the right place and that the Image files are placed in FeralHeart>>Media>>Textures.
Or try...
Downloading it again and replace the files so everything is refreshed, Don't copy make sure you replace them. I had this problem and when I downloaded and replaced the files it worked properly and I was able to change the textures again
I do hope this helped.