coverstack: Do not update the cover on destroy

On rare occasions, the CoverStack can be destroyed while the art is
updated. This can result in an error once the art is retrieved because
the CoverStack does not have children anymore.

gnomemusic/widgets/coverstack.py

@@ -67,6 +67,8 @@ class CoverStack(Gtk.Stack):
         self.props.transition_type = Gtk.StackTransitionType.CROSSFADE
         self.props.visible_child_name = "loading"
 
+        self.connect("destroy", self._on_destroy)
+
         self.show_all()
 
     @GObject.Property(type=object, flags=GObject.ParamFlags.READWRITE)
@@ -134,3 +136,12 @@ class CoverStack(Gtk.Stack):
         self._art = None
 
         self.emit('updated')
+
+    def _on_destroy(self, widget):
+        # If the CoverStack is destroyed while the art is updated,
+        # an error can be coccur once the art is retrieved because
+        # the CoverStack does not have children anymore.
+        if (self._art is not None
+                and self._handler_id is not None):
+            self._art.disconnect(self._handler_id)
+            self._handler_id = None