title: Android Material ButtonToggleGroup get checkedButtonId
body: <p>In last versions meterial ButtonToggleGroup support singleSelection and selectionRequired</p>
  
  <pre><code> implementation 'com.google.android.material:material:1.2.0-alpha06'
  
   app:selectionRequired="true"
   app:singleSelection="true
  </code></pre>
  
  <p>it works well, but when I try to get the id of the already selected button, returns -1</p>
  
  <pre><code>var x = toggleGroup.checkedButtonId
  </code></pre>
  
  <p>How can I get checked button id always?
  I understand that -1 indicates that the current button is already selected. But I have several toggleGroup and I need to know on the fly which buttons id are selected.</p>