Fix crash quitting while details dialog active

* While the details dialog is opened, if quit is triggered, the error message "'DetailsPanel' object has no attribute '_table'" is reported
* A workaround is to cleanly close the dialog before tear down

qt/app.py

@@ -248,6 +248,7 @@ class DupeGuru(QObject):
         preferences_dialog.setParent(None)
 
     def quitTriggered(self):
+        self.details_dialog.close()
         self.directories_dialog.close()
 
     def showAboutBoxTriggered(self):