Uncaught TypeError: Argument 1 passed to Mage_Adminhtml_Controller_Action::_addLeft() must be an instance of Mage_Core_Block_Abstract, boolean given

Question

What does this error mean and how can I fix it? I'm sure it is related to me upgrading to php7. Thanks

Error:

PHP Fatal error:  Uncaught TypeError: Argument 1 passed to Mage_Adminhtml_Controller_Action::_addLeft() must be an instance of Mage_Core_Block_Abstract, boolean given, called in controllers/Adminhtml/ListController.php on line 29 and defined in Mage/Adminhtml/Controller/Action.php:63

From ListController.php:

        public function editAction()
        {
            $id     = $this->getRequest()->getParam('id');
            $model  = Mage::getModel('commentbox/comment')->load($id);
            if ($model->getId() || $id == 0)
            {
                $data = Mage::getSingleton('adminhtml/session')->getFormData(true);
                if (!empty($data))
                    $model->setData($data);

                Mage::register('comment_data', $model);
                $this->loadLayout();
                $this->_setActiveMenu('system/commentbox');
                $this->_addContent($this->getLayout()->createBlock('commentbox/adminhtml_list_edit'))
  line 29-->         ->_addLeft($this->getLayout()->createBlock('commentbox/adminhtml_list_edit_tabs'));
                $this->renderLayout();
            }
            else
            {
                Mage::getSingleton('adminhtml/session')->addError(Mage::helper('commentbox')->__('Specified Script was not found!'));
                $this->_redirect('*/*/');
            }
        }

From Action.php:

line 63-->   protected function _addLeft(Mage_Core_Block_Abstract $block)
    {
        $this->getLayout()->getBlock('left')->append($block);
        return $this;
    }

Answer 1

Based on the comments...
The error appears because the block invoked [Namespace]/[Module]/Block/Adminhtml/List/Edit/Tabs.php does not exist.
In this case the createBlock method returns false and the _addLeft method gives an error because it expects an object as parameter not a boolean value.