title: How to synchronize ElTreeItem with its checkbox(Radiobutton)?
body: <p>Well, there is some problem I am having with this element.</p>
  
  <p>Let ElTreeItem ShowCheckBox be true.</p>
  
  <p>Click the ElTreeItem CheckBox(Radiobutton).</p>
  
  <p><img src="https://i.stack.imgur.com/Iteal.png" alt="enter image description here"></p>
  
  <p>ElTree.Selected is nil(null)</p>
  
  <p>Click the ElTreeItem Text(If its index = 2)</p>
  
  <p><img src="https://i.stack.imgur.com/vlHXu.png" alt="enter image description here"></p>
  
  <p>ElTree.Selected.Index = 2;</p>
  
  <p>Click the ElTreeItem CheckBox(Radiobutton)(If it is in index = 0).</p>
  
  <p><img src="https://i.stack.imgur.com/EBDYK.png" alt="enter image description here"></p>
  
  <p>ElTree.Selected.Index = 2(still 2...)</p>
  
  <p>How can I fix this problem?</p>