GordonLesti/Lesti_Version — pull request #1 by adamkarnowka (2014-06-10 07:07:24+00:00)
Title: Changed base class in Ajax block.

Got following error on 1.9.0.1:
Recoverable Error: Argument 1 passed to Varien_Data_Form_Element_Abstract::addElement() must be an instance of Varien_Data_Form_Element_Abstract, instance of Lesti_Version_Block_Adminhtml_Data_Form_Element_Version_Ajax given, called in /var/www/lib/Varien/Data/Form/Abstract.php on line 148 and defined in /var/www/lib/Varien/Data/Form/Element/Abstract.php on line 55

Changing base class to Varien_Data_Form_Element_Abstract did the trick for me.