title: Cytoscape-popper and Tippy: fails with interactive content
body: <p>I am using cytoscape to display connections in biological literature and want to show references when edges are clicked. </p>
  
  <p>I am following the instructions for using Tippy in conjunction with cytoscape-popper at <a href="https://github.com/cytoscape/cytoscape.js-popper/blob/master/demo-tippy.html" rel="nofollow noreferrer">https://github.com/cytoscape/cytoscape.js-popper/blob/master/demo-tippy.html</a>.  It works fine except when creating href links in the Tippy text, which I'd like to be able to use for click-through.</p>
  
  <p>I attempt to do so by incorporating 'interactive: true' in the code below, but doing so generates a console error <code>TypeError: t is null</code></p>
  
  <pre class="lang-js prettyprint-override"><code>var makeTippy = function(el, text){
    var ref = el.popperRef();
    var dummyDomEle = document.createElement('div');
    var tip = tippy(dummyDomEle, {
      onCreate: function(instance){ // mandatory
        instance.popperInstance.reference = ref;
      },
      lazy: false,
      trigger: 'manual',
      content: function(){
        var div = document.createElement('div');
        div.innerHTML = text;
        return div;
      },
      // own preferences:
      arrow: true,
      placement: 'bottom',
      hideOnClick: true,
      multiple: true,
      //interactive: true,  &lt;-- uncommenting this line generates error
      sticky: true,
      theme: 'run'
    });
  };
  </code></pre>