Laravel registration controller error - Response::setContent() must be of the type string or null

Question

I am experiencing this error when trying to register a user. The registration goes through i suppose because the data is inserted into the database but then it throws up this error instead of redirecting to dashboard.
I am kind of new to this so I am not sure where the error is coming from. I already tried searching for solutions online but not coming across any.

Here is my registerController

protected $redirectTo = RouteServiceProvider::HOME;

    protected function redirectTo()
    {

        $user = auth()->user();

        if ($user->role == 'guest') {
            return route('guest.profile');
        } else if ( $user->role == 'superadmin' ) {
            return route('superadmin.home');
        }

    }

    public function __construct()
    {
        $this->middleware('guest');
    }
    protected function validator(array $data)
    {
        return Validator::make($data, [
            'first_name' => ['required', 'string', 'max:255'],
            'last_name' => ['required', 'string', 'max:255'],
            'email' => ['required', 'string', 'email', 'max:255', 'unique:users'],
            'password' => ['required', 'string', 'min:6', 'confirmed'],
        ]);
    }

    protected function create(array $data)
    {
        return User::create([
            'first_name' => $data['first_name'],
            'last_name' => $data['last_name'],
            'email' => $data['email'],
            'password' => Hash::make($data['password']),
        ]);
    }

Answer 1

Finally seen my error. Was supposed to add an else block to the redirect method. Added it and error went away.

protected function redirectTo()
{
    $user = auth()->user();

    if ($user->role == 'guest') {
        return route('guest.profile');
    } else if ( $user->role == 'superadmin' ) {
        return route('superadmin.home');
    } else {
        return route('home');
    }
}

Thanks all for the help