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

magento, magento-1.7

Solution

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;
    }
}

Problem

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') ) ); ```

Original source