表单容器无法设置子表单/创建表单块

Question

My goal is to have a form into a form container. When I call directly my form in my controller, it works, but when I call the container, I have this issue:

Fatal error: Call to a member function setData() on boolean in app/code/core/Mage/Adminhtml/Block/Widget/Form/Container.php on line 144

The code line is:

public function getFormHtml() {
    $this->getChild('form')->setData('action', $this->getSaveUrl());
    return $this->getChildHtml('form');
}

And this refers to another line:

protected function _prepareLayout() {
    if ($this->_blockGroup && $this->_controller && $this->_mode) {
        $this->setChild('form', $this->getLayout()->createBlock($this->_blockGroup
            . '/'
            . $this->_controller
            . '_'
            . $this->_mode
            . '_form'
            )
        );
    }
    return parent::_prepareLayout();
}

I've tried to tweak values of _blockGroup, _controller, and _mode, but nothing works...

My form container class:

class Cheek_Portraits_Block_Edit extends Mage_Adminhtml_Block_Widget_Form_Container
{
    public function __construct()
        {
            $helper =  Mage::helper('portraits');
            $this->_blockGroup = 'portraits';
            $this->_controller = 'page';
            $this->_mode = 'edit';

            if($this->getRequest()->getParam('id'))
                {
                    $this->_headerText = $helper->__('Edit')." portrait";
                }
            else
                {
                    $this->_headerText = $helper->__('New')." portrait";
                }
            parent::__construct();
        }
}

Here my folders and files:

I have also tried like this: