Fix Flutter Unhandled Exception: This widget has been unmounted, so the State no longer has a context

Widget unmounted? State no longer available? If you’re seeing these types of errors while using Flutter, don’t worry – you’re not alone!

When you see the error Unhandled Exception: This widget has been unmounted, it means that the widget in question – most likely a StatefulWidget – was removed from the tree before the end of its lifetime. There are a few scenarios in which this can happen:

[ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: This widget has been unmounted, so the State no longer has a context (and should be considered defunct).
Consider canceling any active work during "dispose" or using the "mounted" getter to determine if the State is still active.
#0      State.context.<anonymous closure> (package:flutter/src/widgets/framework.dart:909:9)
#1      State.context (package:flutter/src/widgets/framework.dart:915:6)
#2      _SaleOrdersPageState._fetchProducts.<anonymous closure> (package:example/screens/sales/sales_screen.dart:55:9)
#3      _rootRunUnary (dart:async/zone.dart:1436:47)
#4      _CustomZone.runUnary (dart:async/zone.dart:1335:19)
<asynchronous suspension>

• If you’re using setState() to update a widget and the widget is removed from the tree before the end of the setState() callback, you’ll see this error.
• If you’re passing a context to a child widget and the child widget is removed from the tree before the parent widget, you’ll see this error.
• If you’re using a FutureBuilder or StreamBuilder and the Future or Stream completes before the widget is removed from the tree, you’ll see this error.

This issue can be addressed by adding a line of code to check before applying setState method.

if (mounted) { 
  setState (() => _sales = []);
}