InfoPath - LoadDocumentAndPlayEventLog NullReferenceException

Error:

LoadDocumentAndPlayEventLog failed with unhandled exception System.NullReferenceException: Object reference not set to an instance of an object.     at Microsoft.Office.InfoPath.Server.Converter.DetectUnsupportedNamespaces.VerifyNamespace

This is a very uncommon bug.  Essentially, you are using custom code in InfoPath, and you are using code that doesn't have a namespace.  Say a helper function that you've included in your code.

When InfoPath Forms Services attempt to validate your form, it finds that your form template contains reference to code that doesn't have a namespace!

The fix is simple, create a namespace for the helper class, or move the helper class under the namespace of the form's namespace.