magento Call to a member function setProductFilter() on a non-object

It is a Major Problem in Magento 1.9.xx
I found the cause.

If your Problem  is - magento Call to a member function setProductFilter() on a non-object in app/code/core/Mage/Catalog/Model/Product/Type/Configurable.php on line 294


Go to -
app/code/core/Mage/Catalog/Model/Resource/Product/Type/Configurable/Attribute/

And see there are the Collection.php file exist or not.
I thing there are Collection.php file exist but the file extension is not correct format.

You can change the extension only, Make it Collection.php


Thanks