Non-Null check on nested collection mapping

I have a set of classes like the following

ClassRoot, ClassA, ClassB

ClassRootDto, ClassADto, ClassBDto

where

ClassRoot has a list of ClassA, and ClassA has a list of ClassB

ClassRootDto has a list of ClassADto, and ClassADto has a list of ClassBDto

I tried to map a ClassRoot instance to a ClassRootDto instance

The list of ClassADto instances is explicit set to the field of ClassRootDto instance without any problem.

However, when set the list of ClassBDto to the field of a ClassADto instance, the generated code does a null check on the target field.

Why are there different null-check behaviors for different level of mappings?

My current solution:
Initialize an empty ClassBDto list in ClassADto field

My expectation
I expect there should be no null-check on target field when doing collection mapping

Start by reproducing the nested ClassRoot, ClassA, and ClassB mapping described in the issue, then inspect the generated mapper where the ClassBDto collection is assigned. Compare that generated code with the ClassADto collection assignment and determine whether the target null check should differ. Done means the expected behavior is clarified and covered by a regression test.