In my project the FP Gun shows up under mesh, but it is empty and uneditable:
When I drag this blueprint into the editor it also has an error with the FP Gun mesh:
But the old character from this blueprint that existed from the start still works normally:
This project was started with the C++ first person template and then some changes were added to it. I attempted to change the skeletal mesh weapon to my own weapon model in both the character class and the data blueprint then I attempted to revert to the old project mesh.