title: addTools is giving exception in rappidjs
body: <p>In context of one of my assignment I was trying to use <a href="http://jointjs.com/" rel="nofollow noreferrer">RappidJS</a>. I was trying the below code for connecting an element by dropping it over another element. 
  <a href="https://resources.jointjs.com/tutorial/connecting-by-dropping" rel="nofollow noreferrer">https://resources.jointjs.com/tutorial/connecting-by-dropping</a>
  But I am getting exception as below  </p>
  
  <pre><code>rappid.min.js:14 Uncaught TypeError: Cannot read property 'start' of null
  at child.update (rappid.min.js:14)
  at child.onRender (rappid.min.js:14)
  at child.e.render (rappid.min.js:14)
  at child.configure (rappid.min.js:14)
  at child.addTools (rappid.min.js:14)
  at child.element:pointerup (diagram.js:106)
  at triggerEvents (backbone.js:338)
  at triggerApi (backbone.js:322)
  at eventsApi (backbone.js:110)
  at child.Events.trigger (backbone.js:312)
  </code></pre>
  
  <p>I also took reference from below link
  <a href="https://resources.jointjs.com/tutorial/link-tools" rel="nofollow noreferrer">https://resources.jointjs.com/tutorial/link-tools</a> .
  Can any one suggest me what wrong I am doing here.
  My code snippet is as below </p>
  
  <pre><code> function attachLinks(paper) {
                  paper.on({
              'element:pointerdown': function (elementView, event) {
                  event.data = elementView.model.position();
              },
              'element:pointerup': function (elementView, event, x, y) {
                  var coordinates = new joint.g.Point(x, y);
                  var elementAbove = elementView.model;
                  var elementBelow = this.model.findModelsFromPoint(coordinates).find(function (el) {
                      return (el.id !== elementAbove.id);
                  });
                  // If the two elements are connected already, don't
                  // connect them again (this is application-specific though).
                  if (elementBelow &amp;&amp; self.graph.getNeighbors(elementBelow).indexOf(elementAbove) === -1) {
  
                      // Move the element to the position before dragging.
                      elementAbove.position(event.data.x, event.data.y);
                      // Create a connection between elements.
                      var link = new joint.shapes.standard.Link();
                      link.source(elementAbove);
                      link.target(elementBelow);
                      link.addTo(this.model);
                      // Add remove button to the link.
                      var removeLinkTool = new joint.linkTools.Remove();
  
                      var tools = new joint.dia.ToolsView({
                             tools: [ removeLinkTool]
                      });
  
                      var linkView = link.findView(this);
                      linkView.addTools(tools); // getting exception at this place.
                  }
              }
          });
          }
  </code></pre>