Notice: undefined index in wordpress widget

I have done this widget. In the admin area, when I change the value of the variables number, small and large are not saved. And in the frontend i have this notice:

Notice: Undefined index: number in /home/masqueci/public_html/wp-content/themes/Flatnews/includes/theme-widgets.php on line 272

The same for all variables except the title variable.

This is the widget code:

/* CUSTOM TAG CLOUD */
class Tag_cloud extends WP_Widget {

    function Tag_cloud() {
        /* Widget settings. */
        $widget_ops = array( 'classname' => 'Tag_cloud', 'description' => __('Display tag cloud.', 'fabulous') );
        /* Widget control settings. */
        $control_ops = array( 'width' => 200, 'height' => 350, 'id_base' => 'tag_cloud' );
        /* Create the widget. */
        $this->WP_Widget( 'tag_cloud', __('Fabulous tag cloud', 'fabulous'), $widget_ops, $control_ops );
    }

    function widget( $args, $instance ) {
        extract( $args );
        /* User-selected settings. */
        $title = apply_filters('widget_title', $instance['title'] );
        $number = $instance['number'];
        $small = $instance['small'];
        $large = $instance['large'];

        /* Before widget (defined by themes). */
        echo $before_widget;

        /* Title of widget (before and after defined by themes). */
        if ( $title )
            echo $before_title . '<i class="fa fa-tags"></i>' . $title . $after_title;

            echo fab_show_tags ($small, $large, $number);

        /* After widget (defined by themes). */
        echo $after_widget;
    }

    function update( $new_instance, $old_instance ) {
        $instance = $old_instance;
        /* Strip tags (if needed) and update the widget settings. */
        $instance['title'] = strip_tags( $new_instance['title'] );
        $instance['number'] = strip_tags( $new_instance['number'] );
        $instance['small'] = strip_tags( $new_instance['small'] );
        $instance['large'] = strip_tags( $new_instance['large'] );

        return $instance;
    }
    function form( $instance ) {
        /* Set up some default widget settings. */
        $defaults = array( 'title' => 'TAG CLOUD', 'number' => '10', 'small' => '8', 'large' => '12');
        $instance = wp_parse_args( (array) $instance, $defaults );
     ?>
<p>
  <label for="<?php echo $this->get_field_id( 'title' ); ?>">
    <?php _e('Title:', 'fabulous') ?>
  </label>
  <input id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" value="<?php echo $instance['title']; ?>" style="width:100%;" />
</p>

<p>
  <label for="<?php echo $this->get_field_id( 'number' ); ?>">
    <?php _e('Number:', 'fabulous') ?>
  </label>
  <input id="<?php echo $this->get_field_id( 'number' ); ?>" name="<?php echo $this->get_field_name( 'number' ); ?>" value="<?php echo $instance['number']; ?>" style="width:100%;" />
</p>

<p>
  <label for="<?php echo $this->get_field_id( 'small' ); ?>">
    <?php _e('Smallest:', 'fabulous') ?>
  </label>
  <input id="<?php echo $this->get_field_id( 'small' ); ?>" name="<?php echo $this->get_field_name( 'small' ); ?>" value="<?php echo $instance['small']; ?>" style="width:100%;" />
</p>

<p>
  <label for="<?php echo $this->get_field_id( 'large' ); ?>">
    <?php _e('Largest:', 'fabulous') ?>
  </label>
  <input id="<?php echo $this->get_field_id( 'large' ); ?>" name="<?php echo $this->get_field_name( 'large' ); ?>" value="<?php echo $instance['large']; ?>" style="width:100%;" />
</p>

<?php
    }

}

I dont know what is bad, i have done other widgets and i haven´t had this problem i think i have write the same code. Any help this?