CPQ error 'Attempt to de-reference a null object' when a contract with legacy data is renewed or amended

If Subscription records are created using a data import or a custom trigger, you will need to populate specific fields on your Contracts, Subscriptions, and Assets in order to renew or amend the Contract successfully with correct data on the Amendment or Renewal Quote.
 

An "Attempt to De-Reference a Null Object" exception might occur if certain required fields are missing. Unexpected behavior such as missing Quote Lines on the Amendment or Renewal Quote might occur if required fields are missing (such as the Number field or Root ID field on Subscriptions).