title: Kendo UI treeview get locked along with Checkbox
body: <p>when I use <strong>isChecked</strong> function in Treeview all checkboxes are locked and can't change</p>
  
  <pre><code>public data: any[] = [
   { id: 1, isChecked: true, text: 'Furniture' },
   { id: 2, isChecked: false, text: 'Tables &amp; Chairs' },
   { id: 3, isChecked: true, text: 'Sofa 1' },
   { id: 4, isChecked: false, text: 'Sofa 2' }
  ];
  
  public isChecked = (dataItem: any, index: string): CheckedState =&gt; {
    if (dataItem.isChecked === true) {
     return 'checked';
    }
   return 'none';
  }
  </code></pre>
  
  <p>HTML: </p>
  
  <pre><code>&lt;kendo-treeview [nodes]="data" textField="text" kendoTreeViewExpandable kendoTreeViewCheckable [checkBy]="'id'" [isChecked]="isChecked"&gt;
  &lt;/kendo-treeview&gt;
  </code></pre>
  
  <p><a href="https://stackblitz.com/edit/angular-bgqyvr?file=src%2Fapp%2Fapp.component.html" rel="nofollow noreferrer">demo</a></p>