js/main: Unset input method on shutdown

After shutdown, the Javascript context will be disposed, so we must make
sure we remain the active input method after this point.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6535
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2722>

js/ui/main.js

@@ -241,6 +241,8 @@ function _initializeUI() {
 
     inputMethod = new InputMethod.InputMethod();
     Clutter.get_default_backend().set_input_method(inputMethod);
+    global.connect('shutdown',
+        () => Clutter.get_default_backend().set_input_method(null));
 
     screenshotUI = new Screenshot.ScreenshotUI();