Looking up a deactivated widget's ancestor is unsafe.
At this point the state of the widget's element tree is no longer stable. To safely refer to a widget's ancestor in its dispose() method, save a reference to the ancestor by calling inheritFromWidgetOfExactType() in the widget's didChangeDependencies() method
This line of code throws an exception in emulator...
Navigator.pushReplacementNamed(context, '/topics');
Looking up a deactivated widget's ancestor is unsafe. At this point the state of the widget's element tree is no longer stable. To safely refer to a widget's ancestor in its dispose() method, save a reference to the ancestor by calling inheritFromWidgetOfExactType() in the widget's didChangeDependencies() method