Symptoms

Null reference exception when editing form rules in Designer after altering SmartObject.
 

Diagnoses

A SQL stored procedure had been changed during development of the application. The SmartObjects were being created from the SmartObject Service Tester by right clicking on the item in the Service Instance and choosing 'Create SmartObject' every time a change was made. This caused new SmartObjects to be created each time and the existing SmartObject was no longer valid for the refreshed Service Instance or the underlying database.
 

Resolution

Choose 'Generate SmartObjects' by right clicking on the Service Instance. This will let K2 automatically manage the changes of the SmartObjects and the forms/views will no longer experience the described errors.