Object not set to instance to instance of an object with CoC


I am trying to use a chain of command for the Update() method of InventTable.  When I try to compile the code, I receive:

Abnormal termination with unhandled exception. System.NullReferenceException: Object not set to an instance of an object...

The code is below.

public final class InventTable_Table_Extension
    public void Update() 
        ItemTestGroupId ItemTestGroupId = ItemTestGroupTable::FindOrCreateId(this);
        if (this.ItemTestGroup != ItemTestGroupId)
            this.ItemTestGroup = ItemTestGroupId;
        next Update();

Is there something about the Update method that is different from others?