I have a custom object with Opportunbity lookup - not required. I could save a record without a value in Opportunity. Now implementing approval process, i get a flow error, and I have to enter a opportunity value in order to save it, where did that come from?

Error element myRule_1_A1 (FlowRecordUpdate).
This error occurred when the flow tried to update records: The flow failed to access the value for myVariable_current.Opportunity__r.AccountId because it hasn't been set or assigned..

Maybe the Oppportunity was a master-child before? its just a lookup now - thanks