VSTO MailItem.Save error “The operation cannot be performed because the message has been changed” - vb.net

Question

I am trying to change categories color of a current selected mail in outlook 2013 using the explorer object to get the current selected item. Everything seems to be well except when I save it gives the error mentioned above. I have been looking for solutions and no luck any ideas? Thanks. here is my code in VB
Private Sub exp_SelectionChange() Handles exp.SelectionChange ' errrrorr
Try
waitapprovemail = Application.Session.GetItemFromID(exp.Selection.Item(1).EntryID)
if (CheckForRedCategory(waitapprovemail)) Then
If (CheckToReleaseMail(waitapprovemail)) Then
waitapprovemail.Categories = "Green Category"
waitapprovemail.Save() ''' this gives the error
End If
End If
Catch Exc As System.Runtime.InteropServices.COMException
MsgBox(Exc.Message & " " & Exc.Source)
Catch exc As System.InvalidCastException
MsgBox("Casting problem")
End Try
End Sub
Private Function CheckToReleaseMail(mail As MailItem) As Boolean ' errrrrr
' check the id with the ids in the locked mail, if found id then check the other flag if it is false or true, if found true then set the category of that waitemail to empty "" else keep it
Dim r As Boolean = True
Dim sarray As String()
' ofile2 = fso2.OpenTextFile("C:\Users\" & userName & "\Documents\Outlook Files\LockedMail.txt", 8, True) '8 for appending in arg2 0 for tristatefalse optional opens as ascii
Try
Using sr As New StreamReader("C:\Users\" & userName & "\Documents\Outlook Files\LockedMail.txt")
Dim line As String
Do
line = sr.ReadLine()
If (line.Equals("") Or line Is Nothing) Then
r = True
Continue Do
Else
sarray = line.Split(",")
If (sarray.Count > 0) Then
If (sarray(0).Equals(mail.EntryID, StringComparison.InvariantCultureIgnoreCase)) Then
r = False
sr.Close()
mail.Close(OlInspectorClose.olDiscard)
Return r
End If
End If
End If
Loop Until line Is Nothing
sr.Close()
End Using
Catch exc As System.Exception
End Try
mail.Close(OlInspectorClose.olDiscard)
Return r
End Function
Private Function CheckForRedCategory(mail As MailItem) As Boolean ' errrrrr
Dim b As Boolean = False
Try
If (mail.Categories.Equals("Red Category")) Then
b = True
mail.Close(OlInspectorClose.olDiscard)
Return b
Else
b = False
End If
Catch exc As System.NullReferenceException
b = False
mail.Close(OlInspectorClose.olDiscard)
End Try
mail.Close(OlInspectorClose.olDiscard)
Return b
End Function