Salesforce lookup fields

I have an sf object that has lookup fields to other objects. Sometimes these lookups are populated and some times they are not. The issue I am having is that I get an error if I map this lookup field and it is not populated. This is causing me to have to create two to handle where it is populated (leave the fields mapped) and one to handle when it is not (be sure the field is not mapped). This is very time consuming, especially when one object can have 4-5 lookup fields. I am wondering if there is a better way to handle this?

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