title: Extjs syncing a store gives me a url is undefined error under specific conditions
body: <p>I have 4 grids that have the drag and drop plugin enabled. Their initial grid is dependent on a value from the db called state_id.</p>
  
  <p>When I drop the selected row into a new grid, I update the state_id value and then tell it to sync up with the db and update the value for the item in question.</p>
  
  <p><img src="https://i.stack.imgur.com/SBiT7.png" alt="enter image description here"></p>
  
  <p>This works fine for the most part. I get this URL is undefined error when the following happens</p>
  
  <ol>
  <li>User clicks drags row A from grid
  1 to grid 2 </li>
  <li>User drags row A from
  grid 2 to grid 1</li>
  <li>ERROR! Url
  undefined.</li>
  </ol>
  
  <p>This error only seems to happen when the first item being added to the grid originally came from the same grid.</p>
  
  <ol>
  <li>User clicks drags row A from grid 1 to grid 2</li>
  <li>User clicks drags row B from grid 2 to grid 1</li>
  <li>User clicks drags row A from grid 2 to grid 1</li>
  <li>Works as intended!</li>
  </ol>
  
  <p>drop event handler inside my controller:</p>
  
  <pre><code>dropit: function (node, data, dropRec, dropPosition) {
      if (node.dragData.records[0].store.$className == "AM.store.BacklogCards")
      {
          data.records[0].set('state_id', 1);
          this.getBacklogCardsStore().sync();
      }
      else if (node.dragData.records[0].store.$className == "AM.store.InprogressCards")
      {
          data.records[0].set('state_id', 2);
          this.getInprogressCardsStore().sync();
      }
      else if (node.dragData.records[0].store.$className == "AM.store.ReviewCards")
      {
          data.records[0].set('state_id', 3);
          this.getReviewCardsStore().sync();
      }
      else
      {
          data.records[0].set('state_id', 4);
          this.getDoneCardsStore().sync();
      }
      //node.dragData.records[0].store.sync();
  },
  </code></pre>
  
  <p>Any ideas on what is causing this and how to fix it?</p>
  
  <p>Thanks</p>