This mixin overrides default setState method and adds a check if widget is still mounted in the widget tree.

safe_set_state_mixin.dart

import 'package:flutter/widgets.dart';

mixin SafeSetStateMixin<T extends StatefulWidget> on State<T> {
  @override
  void setState(VoidCallback fn) {
    if (!mounted) return;

    super.setState(fn);
  }
}