The runtime theme default.tss.asset failed to be loaded from uitk package while importing the package.

Steps:
1 - Create a new project
2 - Add uitk package to the project
3 - Add a new tss using the "Assets/UIToolkit/Default Runtime Theme File" menu entry
4 - Reimport All

Actual outcome:
There is an error stating that default theme cannot be loaded. The importing .tss file is empty has no content, no sub asset.

Expected outcome:
There is no error and default.tss.asset is deep copied and added as sub asset to the created tss file