Symptoms

When editing a form in K2 Designer, an Error "Object reference not set to an instance of an object." appears. 

Diagnoses

This "Object reference not set to an instance of an object." error usually indicates that a Smartobject used by the Form/View could not be found or has an issue it may have been deleted or recreated with a different GUID.

1. As a troubleshooting step, please edit each of the Views that is used by this form there is maybe a view of this form that is also throwing the same "Object reference not set to an instance of an object." error. If so does this view use a Smartobject, and does this Smartobject exist and execute successfully from the SmartObject Servicer Tester Tool (C:Program Files(x86)K2 blackpearlBin).

2. Please also "Enable HostServer Logging" as per the steps below:
http://help.k2.com/kb000298

Replicating this error may give more details as to which Smartobject/GUID it is referencing. 

Resolution 

A Stored Procedure that is executed by this SmartObject in the Form was not updated in this environment; had been updated in the source environment. Updating this Stored Procedure, refreshing the SQL Service Instance and updating it in the SmartObject resolved the issue.