Not sure why this error is occuring, but it happens every time I attempt to double click to select an Opportunity to record a message to when archiving.  After I clear the error I can upload the message to Method, it just gives me this error on Step 2, the selection screen.
The complete error is:
 
iContext=480
sContextInfo=
System.iIvalidCastException: Conversion from type 'DBNull' to type 'String' is not valid.
     at Microsoft.VisualBasic.CompilerServices.Conversions.ToStrong(ObjectValue)
     at OutlookEasyMailAddInn.frmAddToMethod.subLoadOpportunityTab(Int32 EntityRecordID), String strFullName, String strContact, String str EntityType, Int32 intExistingOpportunityNumber)