title: Flutter AlertDialog Navigator pop black screen issue
body: <p>I have a strange problem with AlertDialog in flutter to dismiss the dialog. I was using the below code snippet to close the dialog as mentioned in the flutter documentation. </p>
  
  <pre><code> Navigator.of(dialogContext).pop();
  </code></pre>
  
  <p>But show how it doesn`t work and make the app into the inactive mode and turns into the black screen window. To make it work again, i have to kill the app and restart again. </p>
  
  <p>Here is the complete code for alertdialog in flutter </p>
  
  <pre><code>Future&lt;Null&gt; _showDialogContactDial(context, Contact contactRecord) async {
  
    return showDialog&lt;Null&gt;(
      context: context,
      barrierDismissible: true, // user must tap button!
      builder: (BuildContext dialogContext) {
        return new AlertDialog(
          title: new Text('Confirm Number'),
          content: new SingleChildScrollView(
            child: new ListBody(
              children: &lt;Widget&gt;[
                new TextFormField(
                  maxLines: 1,
                  decoration: new InputDecoration(hintText: 'Number'),
                  keyboardType: TextInputType.number,
                  autofocus: false,
                  initialValue: contactRecord.phoneNumber.number,
                ),
              ],
            ),
          ),
          actions: &lt;Widget&gt;[
            new FlatButton(
              child: new Text(
                'Call',
                style: TextStyle(color: Colors.black),
              ),
              onPressed: () {
                Navigator.of(dialogContext).pop();
  
  
                _launchURL(
                    context);
              },
            ),
            new FlatButton(
              color: Colors.red,
              child: new Text('Close', style: TextStyle(color: Colors.white)),
              onPressed: () {
                Navigator.of(dialogContext).pop();
              },
            ),
          ],
        );
      },
    );
  }
  </code></pre>
  
  <p>I also noticed that it works for one button "call" without any issues but not for the cancel alert dialog as you see in the same code snippet in both button actions. </p>
  
  <p><a href="https://i.stack.imgur.com/ooFwV.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ooFwV.png" alt="dialog"></a>
  <a href="https://i.stack.imgur.com/J2aHA.png" rel="noreferrer"><img src="https://i.stack.imgur.com/J2aHA.png" alt="after the dialogdismiss"></a>
  <a href="https://i.stack.imgur.com/E3eTp.png" rel="noreferrer"><img src="https://i.stack.imgur.com/E3eTp.png" alt="app"></a>
  Help would be appreciated. </p>