title: Unhandled Exception: 'package:flutter/src/widgets/navigator.dart': Failed assertion: line 3499 pos 12: '!_debugLocked': is not true." FLUTTER
body: <p>I work in flutter project, and i have a problem that :
  the project composed from three Pages (splash.dart, login.dart, dashboard.dart), when the app launch the splash.dart page opened, and i passe the new user to login.dart page, and when he put a valid credentials i passe he to dashboard.dart page but in this second passage i got the error below &quot;[ERROR:flutter/lib/ui/ui_dart_state.cc(166)] Unhandled Exception: 'package:flutter/src/widgets/navigator.dart': Failed assertion: line 3499 pos 12: '!_debugLocked': is not true.&quot;
  Also when i want to display an alert to inform the user to put a valid credentials when he put invalid credentials a got the same problem
  thanks in advance,</p>