Problem Converting a Quote to a Sales Order - Error: Duplicate Customer PO

I've found a problem when I try to convert a Quote to a Sales Order in PostBooks 3.8.1. It is an unhandled error: "Duplicate Customer PO: File quotes.cpp, line 321". I think that this is the reason behind it:
In Customer Workbench -> Settings, if the option "Uses Purchase Orders" is checked for a customer, the system will not allow you to convert a quote to a Sales Order without a unique Customer PO being entered for the quote (this field is in the lower left corner of the Quote form).
I think this is how the system should behave.

Unfortunately, xTuple doesn't have error handling in place if a user fails to include a Customer PO.
It would be better to include a message box to indicate that the user needs to enter a Customer PO, or highlight the field as mandatory if the "Uses Purchase Orders" option is turned on.