Widget popup shows only blank page. setCanLoadTinyMce(true) seems to cause the issue

Question

In backend: If I click on "insert widget" on both, with WYSIWYG editor and without, I only get a blank "widget"-popup. If I enable display errors I see:

Fatal error: Call to a member function setCanLoadTinyMce() on a non-object in /www/sites/dev.example.com/files/html/app/code/core/Mage/Adminhtml/Block/Widget/Form/Container.php on line 85 
Fatal error: Undefined class constant 'STRAIGHT_JOIN' in /www/sites/dev.example.com/files/html/lib/Varien/Db/Select.php on line 69 
Fatal error: Class 'Mage' not found in /www/sites/dev.example.com/files/html/app/code/core/Mage/Core/Model/Resource/Session.php on line 108

If I'm gonna comment out line #85 in Container.php the error seems to be gone and the widgets show up.

// Line #84-87 in Container.php
if (Mage::getSingleton('cms/wysiwyg_config')->isEnabled()) { 
    $this->getLayout()->getBlock('head')->setCanLoadTinyMce(true);  // #85
}

Of course that is a very bad solution. Can anyone point me in the right direction what is probably causing the issue? I couldn't find anything or solve the issue without uncomment this line. Thank you so much.

Magento 1.7.0.2 Multistore setup

Answer 1

Seems Magento is not able to find your head block from Container.php file.

Try calling it like this: Mage::app()->getLayout()->getBlock('head')->setCanLoadTinyMce(true);