Magento: How to get magento fieldset from existing form in admin within observer

Question

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.

$form = $observer->getBlock()->getForm();
$fieldset = $form->getFieldset('fieldset_id');
$fieldset->addField('some_field', 'text', array(
    'name' => 'some_field',
    'label' => Mage::helper('cms')->__('Some Field'),
    'title' => Mage::helper('cms')->__('Some Field')
  )
);

Answer 1

Ok I did it. It was my bad. I took wrong block type. Here is the code:

public function onAdminhtmlBlockHtmlBefore(Varien_Event_Observer $observer)
{
    $block = $observer->getBlock();
    if (!isset($block)) return;

    switch ($block->getType()) {
    case 'adminhtml/some_edit_tab_main':
    $form = $block->getForm();
        $fieldset = $form->getElement('base_fieldset');
        $fieldset->addField('some_field', 'text', array(
                'name' => 'some_field',
                'label' => Mage::helper('cms')->__('Some Field'),
                'title' => Mage::helper('cms')->__('Some Field')
            )
        );
        break;
    }
}