无法从动态创建的ckeditor实例获取数据

Question

I created dynamic ckeditor instances using php

while($a<10){
  echo '<textarea name="editor_'.$a.'" id="editor_'.$a.'"></textarea>';
  echo '<button class="add_data" data-element-id="'.$a.'">Add Data</button>';

  echo "<script type='text/javascript'>
                CKEDITOR.replace('editor_".$a.', {                       
                    toolbar: [
                        ['Bold', 'Italic', 'Underline', 'Strike', 'Subscript', 'Superscript'] 
                     ]
                    });
                </script>";
$a = $a+1;
    }

And I use jquery to get data from particular instance of ckeditor

$('.add_data').click(function(){    
    var element_id = $(this).$(this).attr("data-element-id");
    var data = CKEDITOR.instances['editor_'+element_id].getData();  
    $.ajax({
        ...............
    });
    return false;    
});

but problem id I unable to get data from that particular instance. I got an error in Firebug

TypeError: CKEDITOR.instances[('editor_'+element_id)] is undefined
var data = CKEDITOR.instances['editor_'+element_id].getData();  

I am using CKEditor 4.4.7