if else statement for checkbox in wordpress widget won't work

This is my code for widget in plugin:

class shortcodes_widget extends WP_Widget {

function shortcodes_widget() {
    parent::WP_Widget(false, $name = __('t', 't') );

}

function form($instance) {
if($instance) {
 $title = esc_attr($instance['title']);
 $textarea = esc_textarea($instance['textarea']);
 $checkbox = esc_attr($instance['checkbox']);
} else {
 $title = '';
 $textarea = '';
 $checkbox = '';
}
?>

<p>
  <label for="<?php echo $this->get_field_id('title');?>"><?php   _e('t', 't');    ?></label>
 <input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo $title; ?>" />
</p>

<p>
  <label for="<?php echo $this->get_field_id('textarea'); ?>"><?php _e('t', 't'); ?></label>
  <textarea class="widefat" id="<?php echo     $this->get_field_id('textarea'); ?>" name="<?php echo  $this->get_field_name('textarea'); ?>"><?php echo $textarea; ?></textarea>
</p>
  <p>
  <input id="<?php echo $this->get_field_id('checkbox'); ?>"  name="<?php   echo $this->get_field_name('checkbox'); ?>" type="checkbox" value="1" />
   <label for="<?php echo $this->get_field_id('checkbox'); ?>"><?php _e('t', 't'); ?></label>
 </p>

 <?php
 }

   function update($new_instance, $old_instance) {
  $instance = $old_instance;
  $instance['title'] = strip_tags($new_instance['title']);
  $instance['textarea'] = strip_tags($new_instance['textarea']);
   $instance['checkbox'] = strip_tags($new_instance['checkbox']);

 return $instance;
 }

 public function widget( $args, $instance ) {
 extract( $args );
 while(isset($textarea)) { 
 $textarea = $instance['textarea'];  
  $checkbox = $instance['checkbox'];  
 }

   if(isset($checkbox) and $checkbox == '1') { 
   echo '<div class="myclass">'; 

 }
else { 
 echo '<div>';
}

 echo $before_widget;
if (isset($instance['textarea'])) { 
echo do_shortcode($instance['textarea']);   
} 
echo $after_widget;
echo '</div>';
 }
 }
   add_action('widgets_init', create_function('', 'return           register_widget("shortcodes_widget");'));

Problem is this:

       if(isset($checkbox) and $checkbox == '1') { 
     echo '<div class="myclass">'; 

   }
  else { 
   echo '<div>';
  }

It won't echo first value (div and "myclass") if checkbox is checked. Only echoing second value from "else". I had to add isset and while functions to avoid php notices. I tried with one = and without quotes but nothing helps. Any help? Thanks.