Error - 'Conversion from type 'DBnull' to type 'String' is not valid' when posting a transaction

This error suggests that there is a piece of data missing from a field that requires it. To resolve this issue Support will need to replicate the issue using a copy of the database. Therefore please raise a new case and reference the title of this article and please include the steps to replicate the issue and if the database is on premise, details of how to obtain a copy of the database.