title: How to validate dropdown button in flutter?
body: <p>I'm new to flutter development. I'm trying to validate form with drop down but could not. 
  I followed this link for drop down validation.
  <a href="https://github.com/flutter/flutter/issues/6422#issuecomment-262337023" rel="noreferrer">https://github.com/flutter/flutter/issues/6422#issuecomment-262337023</a></p>
  
  <p>The drop down is being auto validated. </p>