title: QML How could i set default option to null of combobox?
body: <p>My combobox like this:</p>
  
  <p><img src="https://i.stack.imgur.com/VU0gC.png" alt="enter image description here"></p>
  
  <p>but i want this:</p>
  
  <p><img src="https://i.stack.imgur.com/zv7JV.png" alt="enter image description here"></p>
  
  <p>My combobox default is first currentText,but i don not want it show any thing when i have not pitch on noting.
  if i set:</p>
  
  <pre><code>displayText: " "
  </code></pre>
  
  <p>It doesn't work which one I choose.
  the value is null.
  what should i do?</p>