flutter 'package:flutter/src/rendering/object.dart': Failed assertion: line 1711 pos 12: '!_debugDoingThisLayout': is not true. code example

Example: Error: Assertion failed: org-dartlang-sdk:///flutter_web_sdk/lib/_engine/engine/navigation/history.dart:284:14 _userProvidedRouteName != null is not true

WillPopScope(
      onWillPop: () async => true,
      child: Scaffold(blah blah)
);