title: Magento: How to get magento fieldset from existing form in admin within observer
body: <p>I'm having problems when trying to get fieldset in existing admin form in observer.
  I'm getting the form but when I try to get fieldset I'm getting nice error:
  Fatal error: Call to a member function addField() on a non-object
  When I add this new field straight to form everything is fine except that it is outside fieldset. </p>
  
  <pre><code>$form = $observer-&gt;getBlock()-&gt;getForm();
  $fieldset = $form-&gt;getFieldset('fieldset_id');
  $fieldset-&gt;addField('some_field', 'text', array(
      'name' =&gt; 'some_field',
      'label' =&gt; Mage::helper('cms')-&gt;__('Some Field'),
      'title' =&gt; Mage::helper('cms')-&gt;__('Some Field')
    )
  );
  </code></pre>