gtkplacesview: Clear row_for_action when row is gone
row_for_action is set when a row is right clicked (upon opening the popup). The previous row_for_action object data is also cleared during this process. If however, the row has disappeared (from an unmount outside of gtkplacesview, such as the sidebar or physical removal), we attempt to set object data on an invalid object. Set row_for_action as a weak pointer to avoid this situation.