Issue
The error message "Unable to get property '_ObjectType' of undefined or null reference" is displayed when a calculated value field with a lookup function evaluates to null. i.e when a lookup() function does not find a referred item or when an item has no value in a field to query value from.
Resolution
A HotFix (2.11.3.1) has been released for the above issue. Contact Nintex Support for a download link.