Running into get_data() null issues when adding a character sprite full or face.
I zipped my current project in case it helps. No rush, everything else works fine.
I reviewed your project and saw where you were wrong.
You have put the root folder for the graphics in the path "res://Graphics/UI". The dialog tries to find the image in that path ("res://Graphics/UI/image_path") but in your case, in "res://Graphics/UI/" the image doesn't exist because it is in "res://Graphics/"
The editor will always try to move the files you use to their correct folders but sometimes it can fail especially when the files are in subfolders within the folders you have defined in the settings
Anyway throughout today or tomorrow I'll try to upload a new version that accepts subfolders and correct a bug I found testing your project that makes a command duplicate when you try to edit it with dobleclick