After deleting a custom subobject with Parent Object ‘Investment’, users receive 500 error when trying to save a project or idea.

Document ID : KB000072676
Last Modified Date : 07/03/2018
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Steps to Reproduce Error 

1) Create New Subobject with ID=nt_sub_inv and Parent Object = Investment 
2) Change ID to field autonumber 
3) Create Text field (2000 character max) 
4) Add Field to Create/Edit view 
5) Open Project 
6) Go to new custom object list 
7) Create an instance with id=sub1 and save 
8) Go to Admin --> Objects 
9) Delete Subobject that was just created. 
You get the Alert: Do you want to delete the objects and items listed below? .. " 
In Associated Items it shows the instance sub1 (nt_sub_inv). 
Click on YES 
10) Go to the Project (or any other project) 
11) Click Save 

Note: Project Obejct doesn't have Partition associated.

Expected Result:
User should not receive 500 Error and should be able to Save Projects

Actual Result:    
500 Error message appears. Can not Save any Project
This corresponds to defect DE29618 and is resolved in CA PPM 15.3