title: Clearing (and Resetting) Material Select
body: <p>I know there's a few questions already on this, but none of the answers seem to work.</p>
  
  <p>I have a series of Angular selects (<code>mat-select</code>). The options chosen in one <code>mat-select</code> effect the options available in subsequent <code>mat-select</code> instances. For instance, if the first <code>mat-select</code> allowed you to choose <code>fruit</code> or <code>vegetable</code>, the second <code>mat-select</code> might have <code>apple</code> and <code>orange</code> if you chose <code>fruit</code> or <code>cucumber</code> and <code>potato</code> if you chose <code>vegetable</code>. </p>
  
  <p>The problem is that if you then go back and change your <code>fruit</code>/<code>vegetable</code> selection, I need to clear the second <code>mat-select</code> (otherwise, it just displays the new item that has the same selected index). </p>
  
  <p>I've tried using <code>patchValue</code> to set the value of the control to null. I've tried calling <code>reset</code> on the control. None of the options I've found seem to work. </p>
  
  <p>Are there any examples of this working to which I can refer?</p>