title: New admob Express Native ads fail with IllegalStateException
body: <p>I've been trying to integrate express native ads to my application. In the express native ads documentation I've read that they work best when displayed with FULL_WIDTH ad size.
  I tried to set my ad size to FULL_WIDTH but it fails with IllegalStateException:</p>
  
  <pre><code>Caused by: java.lang.IllegalStateException: The ad size and ad unit ID must be set before loadAd is called.
  </code></pre>
  
  <p>Here is my xml code:</p>
  
  <pre><code>&lt;com.google.android.gms.ads.NativeExpressAdView
  android:id="@+id/adView"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  ads:adSize="FULL_WIDTH"
  ads:adUnitId="@string/native_ad_unit"/&gt;
  </code></pre>
  
  <p>The ad unit is correct, it works with other ad sizes like 320x150 etc.</p>
  
  <p>Is there something wrong with my implementation?</p>
  
  <p>Cheers</p>