3399815 - Error during placeholder resolving. Null reference was returned from mapping api for resolved values preventing Custom data collection step to trigger - Onboarding

Additional collection step/Custom data collection is not triggered and the following error in the Business Process Engine tile: "Error during placeholder resolving. Null reference was returned from mapping api for resolved values".

"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."

SAP SuccessFactors Onboarding

1. Candidate complete Personal data collection step
2. System doesn't trigger Additional/Custom data collection
3. For the responsible/admin user it is displayed in business process engine tile: "Error during placeholder resolving. Null reference was returned from mapping api for resolved values"

Missing mapping for document templates.

1. Access Document generation - Manage Document Template Mapping
2. In first dropdown leave as "All" and in second dropdown provide form name
3. No mapping is applied either for Mapping type, Base Object, Target field and rule - it can be missed for one of those target or all of them, the problem will occur the same as every entity must be configured and mapped.
4. Map and restart the task on Business Process Engine tile and test with the candidate once again

Screenshot to exemplify step 3: 

onb, obx, mapping, forms, onb2mapping, custom, forms, template, pfs , KBA , LOD-SF-OBX , Onboarding 2.0 , Problem