WordPress – TinyMCE disappears on widget save

I’ve wanted to create a custom widget in WordPress. For better UX, I want to insert a minimal TinyMCE WYSIWYG editor into the widget.

MY PROBLEM

I’ve integrated the WYSIWYG with the wp_editor() function, but when I enter a value and hit the save button, the buttons disappear and the text is white (but still there).

In firebug, following errors are listed :

1. D.hasChildNodes is not a function (editor-template.js, line 1)

2. NS_ERROR_UNEXPECTED: (tiny_mce.js, line 1)

Here’s the code I’ve got so far:

MY CODE

Form function:

public function form( $instance ) {
    //$pages = 

    $settings = array( 
        'media_buttons' => false,
        'tinymce' => array(
            'theme_advanced_buttons1' => 'bold,italic,underline',
            'theme_advanced_buttons2' => '',
            'theme_advanced_buttons3' => ''
        ),
        'textarea_name' => $this->get_field_name( 'description' )
    );

    if ( $instance ) {
        $title = $instance[ 'title' ];
        $page = $instance['page'];
        $description = $instance['description'];
        $image_uri = $instance['image_uri'];
    } else {
        $title = __( 'New title', 'wpb_widget_domain' );
        $page = __( 'New Page', 'wpb_widget_domain' );
        $image_uri = __( 'New Image', 'wpb_widget_domain' );
        $description = __( 'New Description', 'wpb_widget_domain' );
    }
?>
[some code that doesn't matter]
<p>
<label for="<?php echo $this->get_field_id( 'description' ); ?>"><?php _e( 'Description:' ); ?></label><br />
<?php wp_editor( $description, $this->get_field_id( 'description' ), $settings ) ?>
</p>
[some code that doesn't matter]

<?php 
}

Update function:

public function update( $new_instance, $old_instance ) {
    var_dump($new_instance);
$instance = array();
$instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';
    $instance['page'] = ( ! empty( $new_instance['page'] ) ) ? strip_tags( $new_instance['page'] ) : '';
    $instance['image_uri'] = ( ! empty( $new_instance['image_uri'] ) ) ? strip_tags( $new_instance['image_uri'] ) : '';
    $instance['description'] = ( ! empty( $new_instance['description'] ) ) ? strip_tags( $new_instance['description'] ) : '';

    return $instance;
}

One other thing that may be important:
When I dump the $_POST, the value of the description field is not updated.

Anyone?

Thanks in advance.
HS.