Javascript error on bug page when diff has 0 lines

In https://bugs.edge.launchpad.net/lpbuildbot/+bug/508025, one of the linked branches has a merge proposal with a diff of zero lines, and so the link to the diff is, erm, well, it's not a link, it just says "0 lines". This causes a javascript error "TypeError: Cannot call method 'addClass' of null" in the following code:

  var popupdiff=Y.namespace('code.branchmergeproposal.popupdiff');
  function link_popup_diff_onclick(node){
    var a = node.query('a.diff-link');
    a.addClass('js-action');
    var librarian_url = a.getAttribute('href');
    var api_url=node.query('a.api-ref').getAttribute('href');
    a.on('click',function(e){
      e.preventDefault();
      display_diff(a,api_url,librarian_url);
    });
  }

Unfortunately, this error stops execution such that, for example, the status and importance pickers no longer work.