Error with capabilities on CPT

I added a new post type within a pluign, and i receive always this error while adding the featured image to the CPT post:

Notice: Trying to get property of non-object in \mywebsite\wp-includes\capabilities.php</b> on line 1116
Notice: Trying to get property of non-object in \mywebsite\wp-includes\capabilities.php</b> on line 1117
Notice: Trying to get property of non-object in \mywebsite\wp-includes\capabilities.php</b> on line 1117
Notice: Trying to get property of non-object in \mywebsite\wp-includes\capabilities.php</b> on line 1120
Notice: Trying to get property of non-object in \mywebsite\wp-includes\capabilities.php</b> on line 1120

I repaired the database, disabled all other plugins, and enabled twentytwelve.

Nothing fixed.

I read that it is an error in registering the post type, this is my actual code:

public function __construct() {
	//Register the Testimonials post type
	add_action( 'init', array( $this, 'register_testimonials' ) );
	add_action( 'init', array( $this, 'register_tag_tax' ) );

	//Enable and define metaboxes for the portfolios
	add_filter( 'cmb_meta_boxes', array( $this, 'add_meta_boxes' ) );
	add_action( 'admin_init', array( $this, 'load_meta_boxes_class' ) );
}

/**
 * Register testimonials post type.
 * @return void
 */
public function register_testimonials() {
	$labels = array(
		'name'					=> __( 'Testimonials', ES_TESTIMONIALS_SLUG ),
		'singular_name'			=> __( 'Testimonial', ES_TESTIMONIALS_SLUG ),
		'menu_name'				=> __( 'Testimonials', ES_TESTIMONIALS_SLUG ),
		'all_items'				=> __( 'All Testimonials', ES_TESTIMONIALS_SLUG ),
		'add_new'				=> __( 'Add New', ES_TESTIMONIALS_SLUG ),
		'add_new_item'			=> __( 'Add New Testimonial', ES_TESTIMONIALS_SLUG ),
		'edit_item'				=> __( 'Edit Testimonial', ES_TESTIMONIALS_SLUG ),
		'new_item'				=> __( 'New Testimonial', ES_TESTIMONIALS_SLUG ),
		'view_item'				=> __( 'View Testimonial', ES_TESTIMONIALS_SLUG ),
		'search_items'			=> __( 'Search Testimonials', ES_TESTIMONIALS_SLUG ),
		'not_found'				=> __( 'No testimonials found', ES_TESTIMONIALS_SLUG ),
		'not_found_in_trash'	=> __( 'No testimonials found in trash', ES_TESTIMONIALS_SLUG )
	);

	$args = array(
		'labels' 				=> $labels,
		'description'			=> __( 'Here you can create testimonials for your website.', ES_TESTIMONIALS_SLUG ),
		'public'				=> true,
		'publicly_queryable' 	=> true,
		'show_ui'				=> true,
		'menu_position'			=> 48,
		'menu_icon'				=> 'dashicons-groups',
		'capability_type'		=> apply_filters( 'es_testimonials_capability_type', 'page' ),
		'supports'				=> array( 'title', 'editor', 'excerpt', 'thumbnail' ),
		'rewrite'				=> array( 'slug' => apply_filters( 'es_testimonials_rewrite', 'testimonial' ) )
	);

	register_post_type( 'testimonial', $args );
}

Do you have any idea about what is the problem?