Split shipment: Can not complete the second shipment in Commerce Manager

Steps to reproduce

Precondition: Paypal is configured for Commerce site with payment action is "Authorization"

1. Add two or more items to cart then checkout by Paypal successfully

2. Go to Commerce Manager then split to two shipments

3. Complete shipment #1 successfully

4. Try complete shipment #2

Observed: Can not complete shipment with message ' Object reference not set to an instance of an object."

Expected: Can complete all shipment