Does this issue occur if you import the Tull.puppet file into a new project, or just an existing project?
Does the puppet show up in the Project panel after dismissing the error message?
Another thing to try: 1. Quit Character Animator, then wait ~10 seconds. 2. Relaunch Character Animator and immediately hold down the Shift key while it's launching. This prevents any scenes/puppets from opening in their respective panels. 3. Try importing Tull.puppet again.