Since 1.11.8 (and I think 1.11.7) every time I open a dmg file it causes Finder to retain a ghost window pane (just enough to see the red amber green buttons) behind the opened DMG window. You can close the dmg window but the buttons on the ghost do nothing.
From that point the only way to open finder is to
‘NEW FINDER WINDOW’ as double clicking does nothing.
Only way to clear this is to restart Finder. I can reproduce easily on 10.14.3 and 1.11.8.