Php – An exception has been thrown during the rendering of a template ("Object of class Closure could not be converted to string")

I’m trying to set by default the username of the user by default in a empty field is the field is empty but I get an error message when I try do so.

->add('username', TextType::class, [
'label' => 'username',
 data'=> function(User $user){
return is_null($user->getUsername()) ? $user->getUsername(): "";}

I’ve tried to do it differently like this way but doesn’t do anything.

$builder->add('username, TextType::class, [
    'attr' => [
        'class' => 'form-control form-opacity',
        'autofocus' => true
    ],
    'data' => (is_null($builder->getData()->getUsername()) ? $builder->getData()->getUsername() : "username"
]);