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)),
),
),
);
},
```

首先根据提供的 WebviewScaffold 代码复现报告的导航流程，然后在打开 webview 并点击 AppBar 返回按钮时检查 broadcast_stream_controller.dart。完成的标准是返回操作不再产生报告的异常；该 issue 未提及测试文件。

由索引模型根据 Issue 内容生成。