I've to forms frmPrint & frmEmail and a dataset(MyDataset) with some DataTable and DataTable Adapters. In frmPrint I've the following Sub

Public Sub StartPrinting()
    try
        adapterLettersInSchedules.Fill(ds.LettersInSchedules)
        adapterLetters.Fill(ds.Letters)
        adapterClients.Fill(ds.Clients)
        adapterPrintJobs.GetPrintJobsDueToday(ds.PrintJobs, False, DateTime.Today)

        For Each prow As MyDataSet.PrintJobsRow In ds.PrintJobs
            Dim lisrow As MyDataSet.LettersInSchedulesRow = ds.LettersInSchedules.FindByID(prow.LetterInScheduleID)
            If lisrow.Isemail = False Then
                Dim clientrow As MyDataSet.ClientsRow = ds.Clients.FindByClientID(prow.ClientID)
                Dim letterrow As MyDataSet.LettersRow = ds.Letters.FindByID(lisrow.LetterID)
                'prow.
                'lisrow.is
                Label1.SuspendLayout()
                Label1.Refresh()
                Label1.Text = "Printing letter"

                txt.Rtf = letterrow.LetterContents

                txt.Rtf = txt.Rtf.Replace("<%Firstname%>", clientrow.FirstName)
                txt.Rtf = txt.Rtf.Replace("<%Lastname%>", clientrow.LastName)
                txt.Rtf = txt.Rtf.Replace("<%Title%>", clientrow.Title)
                txt.Rtf = txt.Rtf.Replace("<%Street%>", clientrow.Street)
                txt.Rtf = txt.Rtf.Replace("<%City%>", clientrow.City)
                txt.Rtf = txt.Rtf.Replace("<%State%>", clientrow.State)
                txt.Rtf = txt.Rtf.Replace("<%Zip%>", clientrow.Zip)
                txt.Rtf = txt.Rtf.Replace("<%PhoneH%>", clientrow.PhoneH)
                txt.Rtf = txt.Rtf.Replace("<%PhoneW%>", clientrow.PhoneW)
                txt.Rtf = txt.Rtf.Replace("<%Date%>", DateTime.Today.ToShortDateString)

                Try
                    PDoc.PrinterSettings = printDlg.PrinterSettings
                    PDoc.Print()
                    prow.Printed = True

                    adapterPrintJobs.Update(prow)
                Catch ex As Exception
                End Try
            End If
        Next prow
        ds.PrintJobs.Clear()
    Catch ex As Exception
        MessageBox.Show(ex.Message, "Print", MessageBoxButtons.OK, MessageBoxIcon.Error)
    End Try
End Sub

And in frmEmail i've the Following Sub

Public Sub SendEmails()
    try
        adapterLettersInSchedules.Fill(ds.LettersInSchedules)
        adapterLetters.Fill(ds.Letters)
        adapterClients.Fill(ds.Clients)
        adapterEmailJobs.GetEmailJobsDueToday(ds.EmailJobs, False, Today)
        Dim ls_string As String
        For Each prow As MyDataSet.EmailJobsRow In ds.EmailJobs
            Dim lisrow As MyDataSet.LettersInSchedulesRow = ds.LettersInSchedules.FindByID(prow.LetterInScheduleID)
            If lisrow.Isemail = True Then
                Dim clientrow As MyDataSet.ClientsRow = ds.Clients.FindByClientID(prow.ClientID)
                Dim letterrow As MyDataSet.LettersRow = ds.Letters.FindByID(lisrow.LetterID)

                txt.Rtf = letterrow.LetterContents
                ls_string = RTF2HTML(txt.Rtf)
                ls_string = Mid(ls_string, 1, Len(ls_string) - 176)
                If ls_string = "" Then Throw New Exception("Rtf To HTML Conversion Failed")
                Label1.SuspendLayout()
                Label1.Refresh()
                Label1.Text = "Sending Email"
                If SendEmail(clientrow.Email, ls_string, letterrow.EmailSubject) Then
                    Try
                        prow.Emailed = True

                        adapterEmailJobs.Update(prow)
                    Catch ex As Exception
                    End Try
                Else
                    prow.Emailed = False

                    adapterEmailJobs.Update(prow)
                End If
            End If
        Next prow
    Catch ex As Exception
        MessageBox.Show(ex.Message, "Email", MessageBoxButtons.OK, MessageBoxIcon.Error)
    End Try
End Sub

I'm running this two subs using two different Threads.

Public th As New Thread(New ThreadStart(AddressOf StartFirstPrint))
Public th4 As New Thread(New ThreadStart(AddressOf sendFirstEmail))

Here is the code of StartFirstPrint and sendFirstEmail

Public Sub StartFirstPrint()
    Do While thCont
        Try
            Dim frm As New frmPrint()
            'frm.MdiParent = Me
            frm.StartPrinting()
        Catch ex As Exception

        End Try
    Loop
End Sub



Public Sub sendFirstEmail()
    Do While thCont
        Try
            Dim frmSNDEmail As New frmEmail
            frmSNDEmail.SendEmails()
        Catch ex As Exception

        End Try
    Loop
End Sub

the thCont is a public boolean variable that specifies when to shop those threads.

Most Of the time this works very well. But some times it gives errors Like the following image

I don't know why is this occurring. Please help me.