Expected argument of type "string", "null" given at property path

Actually when I try to edit the form by sending empty fields, the above error comes on ,

My UserType class looks like:

class UserType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('firstName', null, [
                'label' => 'Prénom'
            ])
            ->add('lastName', null, [
                'label' => 'Nom'
            ])
            ->add('email', EmailType::class, [
                'label' => 'Adresse e-mail'
            ])
            ->add('password', PasswordType::class, [
                'label' => 'Mot de passe'
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => User::class,
        ]);
    }
}

Deprecated Solution: This problem can be resolved by adding 'empty_data' param in the builder add function:

So the new UserType classe becomes:

class UserType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('firstName', null, [
                'label' => 'Prénom',
                **'empty_data' => ''**
            ])
            ->add('lastName', null, [
                'label' => 'Nom',
                **'empty_data' => ''**
            ])
            ->add('email', EmailType::class, [
                'label' => 'Adresse e-mail',
                **'empty_data' => ''**
            ])
            ->add('password', PasswordType::class, [
                'label' => 'Mot de passe',
                **'empty_data' => ''**
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => User::class,
        ]);
    }
}

Updated Solution: PHP7.4 comes with typed properties which something powerful, so you can simply declare any property and affect NULL as initial value

Example:

class User {
     
   private ?string $firstName = null;

   public function getFirstName(): ?string {
      return $this->firstName;
   }
   
   public function setFirstName(?string $firstName): self {
      $this->firstName = $firstName;

      return $this;
   }
}