As it happens, the “Transform to…” dialog somehow popped up on top of the main wireframes pane, maybe due to an illicit context menu action, and stayed.
Changing from navigator to thumbnail grid, it disappeared, but unfortunately, on changing back it was there again. It persisted over wireframe changes. The escape key, a test transform, nothing helped to rid this windows except for closing the entire application.
Now I cannot easily reproduce. But maybe - however rare the circumstances are - there should be some means to close the dialog in any means.
This is Wireframes for Desktop 4.0.28 on Win 10 Enterprise 1809.