email notification in laravel 5.5 i get this error ```Trying to access array offset on value of type null```

i want to get email notification each time a new user is registered but after creating php artisan make:notification Taskcompleted and added Notification::route('mail','admin@gmail.com')->notify(new TaskCompleted()); like this in my contoller

public function store(Request $request){
        $employee = request()->validate([
            'employee_id' => 'required|max:250',
            'name' => 'required|max:100',
            'place_of_birth' => 'nullable|max:100',]);
Notification::route('mail','admin@gmail.com')->notify(new TaskCompleted());

i keep getting this error Trying to access array offset on value of type null i have imported the necessary class and configured my .env file with mailtrap,still yet same error

via Chebli Mohamed