How to add optional mobile field in registration form

I have created a new PHP file in the plugins folder and added the code:

function add_tml_registration_form_fields() {
	tml_add_form_field( 'register', 'phone', array(
		'type'     => 'tel',
		'label'    => 'Mobile Number',
		'value'    => tml_get_request_value( 'phone', 'post' ),
		'id'       => 'phone',
		'priority' => 15,
	) );
}
add_action( 'init', 'add_tml_registration_form_fields' );

function validate_tml_registration_form_fields( $errors ) {
	if ( empty( $_POST['phone'] ) ) {
		$errors->add( 'empty_phone', '<strong>ERROR</strong>: Please enter your mobile number.' );
	}
add_filter( 'registration_errors', 'validate_tml_registration_form_fields' );

function save_tml_registration_form_fields( $user_id ) {
	if ( isset( $_POST['phone'] ) ) {
		update_user_meta( $user_id, 'phone', sanitize_text_field( $_POST['phone'] ) );
	}
}
add_action( 'user_register', 'save_tml_registration_form_fields' );

But I am getting error. please refer to this image: https://tinyurl.com/y4mdslwx

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