Hi - I was having the same problem as you. It took me hours to notice what I had done wrong, and it looks like you made the same mistake. The solution is to ensure that your files are saved in
\Data\Public\<your mod guid>\GUI
\Data\Public\<your mod guid>\Assests\Textures
It looks like you have yours in the \Mods directory. When you have your atlas in the wrong place, everything seems good until you reload your project.
