WebviewScaffold exception after clicking back icon

After open the webview with an `appBar`, I click on the back button and receive a exception from:

**File**: `broadcast_stream_controller.dart`

**Exception**:


My Code:

```dart
onTap: () {
Navigator.push(
context,
new MaterialPageRoute(
builder: (context) => new WebviewScaffold(
url: widget.recipe.linkUrl,
appBar: new AppBar(title: new Text(widget.recipe.title)),
),
),
);
},
```