title: Extjs 4.1.3 Nested Grid using rowexpander plugin not working
body: <p>Im trying to use the example from sencha and configure rowexpander. When the grid renders the expander is already opened for each row and with nothing inside. When i click on its icon the following error is generated: 
  Uncaught TypeError: Cannot call method 'addCls' of null
  Have anyone else faced this issue ? Can someone post an example of hpw to use rowexpander please.</p>
  
  <pre><code>        plugins : [{
              ptype : 'rowexpander',
              rowBodyTpl : ['&lt;div id="NestedGridRow-{rowId}"&gt;&lt;/div&gt;'],
              pluginId : 'rowexpanderplugin',
              selectRowOnExpand : true,
  
              // this gives each row a unique identifier based on record's "acct_no"
              rowBodyTpl : ['&lt;div id="NestedGrid-{name}" &gt;&lt;/div&gt;'],
  
              // stick a grid into the rowexpander div whenever it is toggled open
              toggleRow : function(rowIdx) {
                  var rowNode = this.view.getNode(rowIdx), row = Ext.get(rowNode), nextBd = Ext.get(row).down(this.rowBodyTrSelector), hiddenCls = this.rowBodyHiddenCls, record = this.view.getRecord(rowNode), grid = this.getCmp(), name= record.get('name'), targetId = 'NestedGrid-' + name;
  
                  if (row.hasCls(this.rowCollapsedCls)) {
                      row.removeCls(this.rowCollapsedCls);
                      this.recordsExpanded[record.internalId] = true;
                      this.view.fireEvent('expandbody', rowNode, record, nextBd.dom);
  
                      if (rowNode.grid) {
                          nextBd.removeCls(hiddenCls);
                          rowNode.grid.doComponentLayout();
                          rowNode.grid.view.refresh();
                      } else {
  
                          // this is the store for the inner grid
                          Ext.create('innsergridstore', {
  
                              autoLoad : {
                                  callback : function() {
  
                                      // create the inner grid and render it to the row
                                      nextBd.removeCls(hiddenCls);
                                      var grid = Ext.create('NestedGrid', {// &lt;-- this is my "inner" grid view
                                          renderTo : targetId,
                                          store : this,
                                          row : row
                                      });
  
                                      rowNode.grid = grid;
  
  
                                      grid.suspendEvents();
                                  }
                              }
                          });
                      }
  
                  } else {
                      row.addCls(this.rowCollapsedCls);
                      nextBd.addCls(this.rowBodyHiddenCls);
                      this.recordsExpanded[record.internalId] = false;
                      this.view.fireEvent('collapsebody', rowNode, record, nextBd.dom);
                  }
              }
          }]
  </code></pre>