Form container can't set child form / create form block

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 also tried like this :

Answer 1

Check if you really created the the Form.php file from Yourmodule/Yournamespace/Block/Adminhtml/Page/Edit/Form.php

please make sure you have this method inside:

    protected function _prepareForm()
    {
        $form = new Varien_Data_Form(
            [
                'id'         => 'edit_form',
                'action'     => $this->getUrl(
                    '*/*/save',
                    [
                        'id' => $this->getRequest()->getParam('id')
                    ]
                ),
                'method'     => 'post',
                'enctype'    => 'multipart/form-data'
            ]
        );
        $form->setUseContainer(true);
        $this->setForm($form);
        return parent::_prepareForm();
    }

For me missing that file caused this error.