When there is a jump between pages, the device return key cannot be heard through WillPopScope

my code:
new WillPopScope(
child:
new WebviewScaffold(
url: url,
appBar: new AppBar(
iconTheme: IconThemeData(
color: Colors.white
),
backgroundColor: Color.fromARGB(255, 41, 58, 144),
title: new Text(title,style: new TextStyle(color: Colors.white),),
),
),
onWillPop: _requestPop);