How To avoid having 'null' ObjectIDs (ie AssignmentID, IncidentID, etc) on object creation.
This problem is generally caused by the ID field being set to allow Null values (Nullable).
The fix is to set the 'Nullable' attribute 'off' by unchecking the checkbox. See image below.
Fix this for 'Task.Assignment':
1. Open Configure Application
2. Open the Business Objects workspace
3. Open the Task.Assignment business object
4. Click the Fields tab
5. Find the AssignmentID field
6. Open the field
7. Find the 'Nullable' checkbox, uncheck it
8. Save your work.