Getting an error while trying to get value from parent BO's Attribute in Before Insert Trigger

Summary:

I am using Before Insert trigger, in which trying to get the value of BO' attribute.

Content (please ensure you mask any confidential information):

For example, I have two BOs. DeptBO and EmpBO with master child relationship. Both BOs have a similar column Location. So the use case is, when ever user didn't put Location in EmpBO then we need to check DeptBO and get the location from there while insert a new record in EmpBO.

For this I have created a Before Insert Trigger on EmpBO as a Business Rule, created a condition when EmpBO.Location is empty