Reminder Agent: Getting "Object Variable not set"

I'm developing a training resources database. There is a form and a hidden view called "Courses2". TThe fields, from the form, in the view used are:
Employee_Email
Trainer_Email
TL_Email
HR_Email
Act_Start_Date

All fields are text except for Act_Start_Date which is Date/Time. All are editable.                                                

I've written a scheduled agent that is supposed to run once a day on documents in a view. The agent checks all the documents in the view and checks that the date in Act_Start_Date is 30 days before todays date. If it finds a document it mails the employee and the TL (team leader).

When I run the agent manually I get "Object Variable not set".

My code is

Sub Initialize
      Dim s As New notessession
      Dim db As notesdatabase
      Set db=s.currentdatabase
      Dim due As notesview
      Dim maildoc, viewdoc As notesdocument
      Dim daten As Variant
      Dim duedate As String
      Dim TL As String
      Dim Employee As String
      Set due=db.getview("Courses2")
      Set viewdoc=due.getfirstdocument
      
      Do Until viewdoc Is Nothing
            
            daten=Today()
            duedate=viewdoc.Act_Start_Date(0)
            TL=viewdoc.TL_Email("")
            Employee=viewdoc.Employee_Email(0)
            
            If daten=(duedate-30) Then
                  Set maildoc=db.createdocument
                  maildoc.form="Memo"
                  Dim rtitem As New notesrichtextitem(maildoc, "Body")
                  maildoc.Subject="Training Reminder - 1 month to go"
                  Call maildoc.AppendItemValue("SendTo",TL)
                  Call maildoc.AppendItemValue("CC",Employee)
                  Call rtitem.AppendText( "Training in a month's time for"& Employee)
                  Call maildoc.send(False, maildoc.SendTo(0))
            End If
            
            Set viewdoc=due.getnextdocument(viewdoc)
      Loop
      
End Sub