Fix crash on shutdown

* Fixes "'DetailsPanel' object has no attribute '_table'" error on shutdown if the Results table is updated (item removed) while the Details Dialog is shown as a floating window.
* It seems that QApplication.quit() triggers some sort of refresh on the floating QDockWidget, which in turn makes calls to the underlying model that is possibly being destroyed, ie. there might be a race condition here.
* Closing or hiding the QDockWidget before the cal to quit() is a workaround. Similarly, this is already done in the quitTriggered() method anyway.
* This fixes #857.

qt/app.py

@@ -271,6 +271,9 @@ class DupeGuru(QObject):
         self.willSavePrefs.emit()
         self.prefs.save()
         self.model.save()
+        # Workaround for #857, hide() or close().
+        if self.details_dialog is not None:
+            self.details_dialog.close()
         QApplication.quit()
 
     # --- Signals

qt/details_dialog.py

@@ -51,7 +51,7 @@ class DetailsDialog(QDockWidget):
             if not self.titleBarWidget():  # default title bar
                 self.setTitleBarWidget(QWidget())  # disables title bar
                 # Windows (and MacOS?) users cannot move a floating window which
-                # has not native decoration so we force it to dock for now
+                # has no native decoration so we force it to dock for now
                 if not ISLINUX:
                     self.setFloating(False)
         elif self.titleBarWidget() is not None:  # title bar is disabled