settings-dialog: Clear matrix_homeserver_dialog object on finalize

Fixes #592

src/dialogs/chatty-settings-dialog.c

@@ -1011,6 +1011,7 @@ chatty_settings_dialog_finalize (GObject *object)
     g_cancellable_cancel (self->cancellable);
   g_clear_object (&self->cancellable);
   g_clear_object (&self->settings);
+  g_clear_object (&self->matrix_homeserver_dialog);
 
   G_OBJECT_CLASS (chatty_settings_dialog_parent_class)->finalize (object);
 }