VB.NET accessing class objects from launched thread - vb.net

Question

I am writing an application in VB.NET that allows users to schedule submissions (emails) to be sent at a later date. I use threads to wait until the time is right to send a particular submission, but for some reason I can't access one of the class objects from the listener threads (or something else is happening, that's what I'm trying to figure out). Here is the relevant code:
Public Class AppContext
Inherits ApplicationContext
Private submsnMngr As SubmissionManager
Public Sub New()
submsnMgr = New SubmissionManager()
menuAddEdit = New ToolStripMenuItem("Add/Edit Submissions")
...
End Sub
Private Sub menuAddEdit_Click(ByVal sender As Object, ByVal e As System.EventArgs)
Handles menuAddEdit.Click
' The user clicking this tray button is the ONLY way that the form can be shown
submsnMngr.ShowWelcome()
End Sub
...
End Class
Class SubmissionManager
Public currentSubmissions As SubmissionList
Public WelcomeForm As Welcome
Public Sub ShowWelcome()
If WelcomeForm Is Nothing Then
' Welcome is the form that needs to be refreshed down in the MailSender subroutine
WelcomeForm = New Welcome(Me)
End If
WelcomeForm.Show()
End Sub
Public Sub CheckDates()
For Each submsn In currentSubmissions.Submissions
SyncLock submsn
If Today.Date >= submsn.EffDate.AddDays(-90).Date And Not submsn.Sent90 And Not submsn.Denied90 And submsn.Thread Is Nothing Then
submsn.Send(1)
submsn.Sent90 = True
currentSubmissions.Save()
ElseIf Today.Date = submsn.EffDate.AddDays(-91).Date And submsn.Thread Is Nothing Then
Dim thd As New Thread(AddressOf MailSender)
thd.IsBackground = True
submsn.Thread = thd
Dim args As New ThreadArgs(submsn.Insured, 1)
thd.Start(args)
End If
If Today.Date >= submsn.EffDate.AddDays(-60).Date And submsn.Thread Is Nothing Then
submsn.Send(2)
currentSubmissions.RemoveSubmission(submsn)
If WelcomeForm IsNot Nothing Then
WelcomeForm.RefreshSubmissions()
End If
ElseIf Today.Date = submsn.EffDate.AddDays(-61).Date And submsn.Thread Is Nothing Then
Dim thd As New Thread(AddressOf MailSender)
thd.IsBackground = True
submsn.Thread = thd
Dim args As New ThreadArgs(submsn.Insured, 2)
thd.Start(args)
End If
End SyncLock
Next
End Sub
Private Sub DateListener()
Do
CheckDates()
Thread.Sleep(3600000)
Loop
End Sub
Private Sub MailSender(args As ThreadArgs)
Dim wait As New TimeSpan(14 - DateTime.Now.Hour, 23 - DateTime.Now.Minute, 0)
Thread.Sleep(wait.TotalMilliseconds)
Dim submsn As Submission = currentSubmissions.GetSubmission(args.insured)
SyncLock submsn
submsn.Send(args.mode)
If args.mode = 1 Then
submsn.Sent90 = True
submsn.Thread = Nothing
currentSubmissions.Save()
Else
currentSubmissions.RemoveSubmission(submsn)
End If
End SyncLock
If WelcomeForm IsNot Nothing Then
' Here is the issue, this code is not being run, even though WelcomeForm is set
' in New() above
WelcomeForm.RefreshSubmissions()
End If
End Sub
End Class
Paying special attention to the few comment lines in the code above, why is WelcomeForm Nothing when I clearly set it to reference the form created in the New() subroutine? I tried alternatively sending the reference to the MailSender thread as an argument, but the same thing happened. Note that I need the If statement there because the user may have closed the form before the thread gets to that point. But it is essential that RefreshSubmissions() be called on it if it is still open.

Answer 1

Sorry guys, realized my thread was being aborted elsewhere in my application's code. No problems with the actual code I posted above.