When running a user sync job via the User Synchronization app with Azure Active Directory, you receive the following error:
"Object reference not set to an instance of an object"

Recreate the entire Azure Active Directory SSO setup by following these 3 KBA articles:
  1. KBA-36306: How to Create an App Registration for DocuWare Single Sign-On
  2. KBA-36308: How to Configure Single Sign-On
  3. KBA-36343: How to Connect DocuWare User Sync Tool with Azure Active Directory
Once the process has been completed, you will be able to utilize User Sync without error.
KBA applicable to Cloud and On-premise Organizations.