Records should be able to be added to the subgrid even if the record is associated with a parent record.

[Actual Results] When adding an existing record to a subgrid, if the record is related to a parent record, the message "This record cannot be added because it already has a parent record." is displayed and cannot be added. [Expected Results] When adding an existing record to a subgrid, the target record can be added to the subgrid without the message above even if it is associated with the parent record.