Avoid crash when quitting

* If details dialog failed to be created for some reason, avoid crashing by dereferencing a null pointer

qt/app.py

@@ -248,6 +248,7 @@ class DupeGuru(QObject):
         preferences_dialog.setParent(None)
 
     def quitTriggered(self):
+        if self.details_dialog is not None:
             self.details_dialog.close()
         self.directories_dialog.close()