Null object cannot be converted to a value type
This is a known issue with ConnectWise PSA. Currently they are working on permanent fix.
ConnectWise PSA suggests performing the following steps in order to resolve this manually:
1. Open PO that is throwing the error.
2. Check the Vendor Company in PO.
3. Open the Vendor company.
4. Switch to Finance tab of the vendor company.
5. Tax code will be empty.
6. Set it to something, it should be fulfilled.
7. Change the status of the PO to Open.
8. Change the status of the PO to Closed.
9. Re-try batching.