Removing a relation sometimes fails

When removing a relation sometimes an exception is raised:

Uncaught TypeError: Cannot call method 'one' of null

It is raised by the setting of relationElement below:

 removeRelation: function(d, context, view, confirmButton) {
      var env = this.get('component').get('env');
      var endpoints = d.endpoints;
      var relationElement = Y.one(context.parentNode).one('.relation');
      utils.addSVGClass(relationElement, 'to-remove pending-relation');
      env.remove_relation(
          endpoints[0][0] + ':' + endpoints[0][1].name,
          endpoints[1][0] + ':' + endpoints[1][1].name,
          Y.bind(this._removeRelationCallback, this, view,
          relationElement, d.relation_id, confirmButton));
    },

To reproduce, go to uistage and attempt to remove the relation between mediawiki and memcached.

Note that repeated tries all result in the same failure. However, after successfully removing another relation, the affected one can sometimes be removed.