title: Material toggle button problem on Android API 21
body: <p>I use material toggle button group like this</p>
  <pre><code>    &lt;com.google.android.material.button.MaterialButtonToggleGroup
          android:layout_width=&quot;wrap_content&quot;
          android:layout_height=&quot;wrap_content&quot;
          app:layout_constraintEnd_toEndOf=&quot;parent&quot;
          app:layout_constraintStart_toStartOf=&quot;parent&quot;
          app:layout_constraintTop_toTopOf=&quot;parent&quot;
          app:singleSelection=&quot;true&quot;&gt;
  
          &lt;com.google.android.material.button.MaterialButton
              android:id=&quot;@+id/btn_topso&quot;
              style=&quot;@style/Widget.MaterialComponents.Button.OutlinedButton&quot;
              android:layout_height=&quot;wrap_content&quot;
  
    android:layout_width=&quot;wrap_content&quot;          android:text=&quot;@string/topo&quot; /&gt;
  
          &lt;com.google.android.material.button.MaterialButton
              android:id=&quot;@+id/btn_averages&quot;
              style=&quot;@style/Widget.MaterialComponents.Button.OutlinedButton&quot;
              android:layout_width=&quot;wrap_content&quot;
              android:layout_height=&quot;wrap_content&quot;
              android:text=&quot;@string/average&quot; /&gt;
  
      &lt;/com.google.android.material.button.MaterialButtonToggleGroup&gt;
  </code></pre>
  <p>But when run the application, crash with this error:</p>
  <pre><code> java.lang.IllegalStateException: Attempted to get ShapeAppearanceModel from a MaterialButton which has an overwritten background.
          at com.google.android.material.button.MaterialButton.getShapeAppearanceModel(MaterialButton.java:1169)
          at com.google.android.material.button.MaterialButtonToggleGroup.addView(MaterialButtonToggleGroup.java:244)
          at android.view.ViewGroup.addView(ViewGroup.java:3709)
          at android.view.LayoutInflater.rInflate(LayoutInflater.java:810)
          at android.view.LayoutInflater.rInflate(LayoutInflater.java:809)
          at android.view.LayoutInflater.rInflate(LayoutInflater.java:809)
          at android.view.LayoutInflater.inflate(LayoutInflater.java:504)
          at android.view.LayoutInflater.inflate(LayoutInflater.java:414)
  </code></pre>
  <p>I can't fix it!, how can I solve this issue?</p>
  <p>Material version: '1.3.0-alpha03'</p>
  <p>Style:</p>
  <pre><code>    &lt;style name=&quot;MaterialAppTheme&quot; parent=&quot;Theme.MaterialComponents.Light.NoActionBar&quot;&gt;
      &lt;item name=&quot;colorPrimary&quot;&gt;@color/primary&lt;/item&gt;
      &lt;item name=&quot;colorPrimaryDark&quot;&gt;@color/primaryDark&lt;/item&gt;
      &lt;item name=&quot;colorAccent&quot;&gt;@color/accent&lt;/item&gt;
      &lt;item name=&quot;android:buttonStyle&quot;&gt;@style/ButtonStyle&lt;/item&gt;
      &lt;item name=&quot;android:background&quot;&gt;@android:color/transparent&lt;/item&gt;
      &lt;item name=&quot;floatingActionButtonStyle&quot;&gt;@style/FloatingButtonStyle&lt;/item&gt;
  &lt;/style&gt;
  </code></pre>