Symptoms

The following error occurs when attempting to edit a particular K2 form:
"Object reference not set to an instance of an object."
 

Diagnoses

We determined that one of the view referenced by this form was giving the same error (usually this can be done by expanding the form in the designer and running each of the view it references, to further narrow down which view may be causing the behavior).

Enabling verbose K2 Hostserver logging and reproducing the error allowed us to find the Smartobject that the view was attempting to call (via its guid and the aSmartBroker].BSmartobject] table in the logs, usually a call is made to retrieve the Smartobject via its GUID a few lines up before the error is logged). Execution of this smartobject via the Tester tool also threw the "Object reference not set to an instance of an object" error. 

Resolution 
We determined that the stored procedure for this SQL service smartobject was incorrect, and as such updated the stored procedure, refreshed the SQL server service instance, and remapped the Smartobject/ServiceObject bindings. This resolved the issue with the Smartobject, View, and Form and allowed for editing again.