Version: (5.99.0-8593-g9f18fee124), release build
As the title states: Assign Footprints hangs when first launched.
Had this problem with the slightly older nightly as well.
I don’t remember the exact steps I used to close the hung windows.
After closing all the windows, and not saving, the Assign Footprints tool launches without issue. I can repeat the issue by closing KiCad and reinstalling it.