SkeletonDebugger isn't really savable. It has some internal variables, such as wires, points and interBoneWires. These can't be null. But after serializing they of course are null...

After loading previously saved scene with a SkeletonDebugger.. updating it will fail on NPE.

I tested with https://sketchfab.com/3d-models/silver-dragonkin-mir4-89ead4e87cdc4b70840f748383f0998f. Looks like the import always creates this debugger.