Symptoms

If you have a Primary Workflow where the start rule is when a list Item Added . From within this workflow if you are triggering a secondary workflow. In the tertiary workflow, if you are using an activity such as Get List Item Reference . When the execution reaches this activity "An Object reference not set to an instance of an object".

Triggering the secondary workflow manually seems to now show the error.
 

Diagnoses

This has been logged as a bug with the way that the Get List Item Reference works. There is a workaround available for the issue:
 

Resolution

A way you can workaround the issue is by

1. In the Child-Process you have a CreateReferece Event, with a Reference Name. In this workaround example it will be referred to as "ChildProcessCreateReference"

2. In the Parent Process, before the Call to the Sub-Process event, drop the Create Reference Wizard and configure it the same as your client process with the same:
* SmartObject,
* Method
* And the ReferenceName: "ChildProcessCreateReference"

This should prevent the Object Reference Error.