Unable to create Assembly Items due to Unexpected Error (WIP Enabled)

When Creating Assembly Items with Costing Method = Standard, the WIP Cost Variance Account is not used and should be null. WIP Cost Variance Account is greyed out if Costing Method = Standard.

If you have a Default WIP Cost Variance Account set under Setup > Accounting > Accounting Preferences > Items /Transactions, it automatically populates when you go to Lists > Accounting > Items > New > Assembly/Bill of Materials. If Costing Method = Standard; Once you click Save, an Unexpected Error will be thrown and Assembly/Bill of Materials Record is not saved.

You need to Go to Purchasing/Inventory Tab > Set Cost Method = Average > go to Accounting Tab > Set WIP Cost Variance Account = Null > Go back to Purchasing/Inventory Tab > Costing Method = Standard > Click Save.