gui_qt: fix possible crash on Linux

If `_NET_ACTIVE_WINDOW` is not supported by the current window manager,
then we can't track the currently focused window and should therefore
not try to restore focus to it.

plover/gui_qt/main_window.py

@@ -169,7 +169,7 @@ def _activate_dialog(self, name, args=(), manage_windows=False):
             def on_finished():
                 del self._active_dialogs[name]
                 dialog.deleteLater()
-                if manage_windows:
+                if manage_windows and previous_window is not None:
                     wmctrl.SetForegroundWindow(previous_window)
             dialog.finished.connect(on_finished)
         dialog.show()