The stacktrace indicates that the following XPath retrieve is executed:
//MxModelReflection.MxObjectMember[AttributeName=Planned_Submission_Date]
This query cannot be executed because of this error:
Object 'MxModelReflection.MxObjectMember' doesn't contain member 'PlannedSubmissionDate'
I think that the XPath you intended to execute should be this (notice the quotes):
//MxModelReflection.MxObjectMember[AttributeName='Planned_Submission_Date']