Reference Joint of Mimic Joint not selected if the reference joint is merged by "Merge FIxed Joints"

[KKSTB]

Hi, thanks for the great plugin.

As the title suggest, if the Merge Fixed Joints option is checked, and the reference joint of the mimic joint is a fixed joint, then the Reference Joint value of the mimic joint of the imported robot is not selected. This is probably because the reference joint is merged into the link and it no longer exists.

Example URDF that has this problem: https://github.com/KKSTB/isaac_lab_gundam_robot_urdf/blob/master/gundam_rx78_description/urdf/GGC_TestModel_rx78_20170112_.urdf, where all the crotch mimic joints under base_link are having the problem after imported to Isaac Sim. It can be tested by cloning the project and importing the URDF.