Magento – Target element not found for content update

First you have to fix a couple of problems with your module. You need to define the model in your config.xml and you have declared your class incorrectly in your Myoption.php file.

In app\code\local\Devshree\Unidesing\Model\Myoption.php change:

Class MyNameSpace_Mymodule_Model_Myoption
extends Mage_Eav_Model_Entity_Attribute_Source_Abstract

To:

class Devshree_Unidesing_Model_Myoption
extends Mage_Eav_Model_Entity_Attribute_Source_Abstract

In app\code\local\Devshree\Unidesing\etc\config.xml replace it with this:

<?xml version="1.0"?>
<config>
  <modules>
    <Devshree_Unidesing>
      <version>0.1.0</version>
    </Devshree_Unidesing>
  </modules>
  <global>
    <models>
        <unidesign>
            <class>Devshree_Unidesing_Model</class>
        </unidesign>
    </models>
    <resources>
      <customcatattrb_setup>
        <setup>
          <module>Devshree_Unidesing</module>
          <class>Mage_Eav_Model_Entity_Setup</class>
        </setup>
        <connection>
          <use>default_setup</use>
        </connection>
      </customcatattrb_setup>
    </resources>
  </global>
</config> 

This happens because you name your field content. Then the TinyMCE js looks for the element with id content to apply the editor. But there is already a content element on the page and it's a div.

Adding a prefix to the form elements solves this issue because the element does not have the id content anymore. It has something_content.
You can add this prefix with one line of code.

$form->setHtmlIdPrefix('something_');

Doing this, you don't need to alter the form data you pass to $form->addValues to add your prefix. It will all be done automatically.

The id prefix can be different for each tab if you have multiple tabs.