Another exception was thrown: 'package:flutter/src/widgets/navigator.dart': Failed assertion: line 2216 pos 12: '!_debugLocked': is not true
when i had experienced this useless error text it was because of screen root widget had no defined / finite size, i had at the root a "SingleChildScrollView",
to solve this i simply wrapped it in Container and set the height to screen max height like this:
@override Widget build(BuildContext context) { return Container( height: MediaQuery.of(context).size.height, child: SingleChildScrollView( child: ... ), ), }