Comment 8 for bug 1729805

I'll check my copy again this evening, but in the mean time there is something simple you could try:

1. Create shape
2. Run extension with default settings
3. Alt+tab to another app
4. Alt+Tab back to Inkscape

If there's windows hidden underneath things this should bring them to the front (at least on my machines), but only if the extension is "hung" with the mystery blank rectangular window open.