Do not use BuildContexts across async gaps

Update Flutter 3.7+ :

mounted property is now officially added to BuildContext, so you can check it from everywhere, whether it comes from a StatefulWidget State, or from a Stateless widget.

While storing context into external classes stays a bad practice, you can now check it safely after an async call like this :

class MyCustomClass {
  const MyCustomClass();

  Future<void> myAsyncMethod(BuildContext context) async {
    Navigator.of(context).push(/*waiting dialog */);
    await Future.delayed(const Duration(seconds: 2));
    if (context.mounted) Navigator.of(context).pop();
  }
}

// Into widget
  @override
  Widget build(BuildContext context) {
    return IconButton(
      onPressed: () => const MyCustomClass().myAsyncMethod(context),
      icon: const Icon(Icons.bug_report),
    );
  }
// Into widget

Original answer

Don’t stock context directly into custom classes, and don’t use context after async if you’re not sure your widget is mounted.

Do something like this:

class MyCustomClass {
  const MyCustomClass();

  Future<void> myAsyncMethod(BuildContext context, VoidCallback onSuccess) async {
    await Future.delayed(const Duration(seconds: 2));
    onSuccess.call();
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  @override
  Widget build(BuildContext context) {
    return IconButton(
      onPressed: () => const MyCustomClass().myAsyncMethod(context, () {
        if (!mounted) return;
        Navigator.of(context).pop();
      }),
      icon: const Icon(Icons.bug_report),
    );
  }
}