title: Show hint text if user is trying to click on disabled Radio button
body: <p>I'd like to implement this feature:</p>
  
  <blockquote>
    <p>When user click on disabled radio buttons, it should display a hint text to tell user that it is not allowed to change choices.</p>
  </blockquote>
  
  <p>I've checked <a href="https://api.flutter.dev/flutter/material/Tooltip-class.html" rel="nofollow noreferrer">Tooltip</a>, it is said:</p>
  
  <blockquote>
    <p>Wrap the button in a Tooltip widget to show a label when the widget long pressed (or when the user takes some other appropriate action).</p>
  </blockquote>
  
  <p>Regarding to Tooltip, I only know how to implement it when user <strong>long pressed</strong> the widget to show tooltip.
  I don't know how to achieve <em>when the user takes some other appropriate action</em> mentioned in above.</p>
  
  <p>In my case, the appropriate action to trigger tooltip is <strong>one click</strong> on the disabled Radio button.</p>
  
  <p>Other things I've tried:</p>
  
  <ul>
  <li><a href="https://stackoverflow.com/questions/50024222/flutter-tooltip-on-all-widgets">Flutter Tooltip on all widgets</a> </li>
  <li><a href="https://stackoverflow.com/questions/57348044/flutter-how-to-update-hint-text-with-current-value-in-textfield">Flutter how to update hint text
  with current value in TextField</a></li>
  </ul>
  
  <p>Still can't figure out.</p>
  
  <p><strong>More Information:</strong></p>
  
  <p>It doesn't have to be Tooltip, please advise any appropriate solution.
  I am new to Flutter/Dart, I hope I have explained the issue clearly.</p>
  
  <p>Thank you.</p>