Are you just deleting the UN dataset from the feature dataset and receiving a crash?
If so, this is different than the bug you are including in your screen captures as they are deleting domains.
I cannot reproduce the crash with your UN provided in the zip file, but I am using ArcGIS Pro 2.8.
1. What if you delete the UN at the feature dataset level?
2. Are you able to reproduce this crash with any FGDB UN that has certain domains configured or is it specific to your FGDB dataset?