Same issue here, when I used it last it worked fine, but when I opened today I can't select sprites with mouse click as mentioned by cheese:3 the menu shows up for a second and then disappears). I can select sprites from right hand 'layers' panel but the left hand menu still doesn't work - I try to delete, unlink, use sliders etc but the menu just disappears and the message Change costume 1 comes up at the bottom.
Anyone has a fix for it?
I'm on MacOS