threading and controls works one way, but not with 2nd method

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VB

I'm having massive troubles understanding threading. Driving me insane.
Could someone please explain why the following won't work.

It has a background thread. I'm trying to get the text different colours which I can get to work if I don't use threading, but when I do it comes up with cross threading.

The part I don't understand is, if I comment out the do --> loop, the textbox is put onto the form. But, if it isn't I get cross threading errors. I don't know why, in my head, I have created the instance of the textbox, and then inputted the text as I require into the textbox and then I invoke the flowlayoutpanel1 to add the textbox control with everything in it.

Private Sub txtboxcreate(ByVal datastring As System.Object, ByRef currenttb As RichTextBox)
        Dim count As Integer = 1
        Dim tb As New RichTextBox
        Name = ("TxtBox" & CStr(counttotal))
        '      Dim text As String = datastring
        With tb
            .Name = Name
            '         .Text = text
            .Multiline = True
            .AutoSize = True
            .Width = 250
            .ReadOnly = True
            .ScrollBars = False
            '       .MaxLength = 100
            .Tag = 20
            .WordWrap = True
            .BackColor = Color.Black

                AddHandler tb.MouseClick, AddressOf Me.TextBox1_MouseClick
        End With
        count = (count + counttotal)
        Dim linecount As Integer = 0
        Dim strreader As New StringReader(datastring)
        Dim line As String
        updatefrm(tb)
        Do
            linecount = linecount + 1
            line = strreader.ReadLine()
            If line.Contains("xred") Then
                tb.SelectionColor = Color.Red
            End If
            If line.Contains("xblk") Then
                tb.SelectionColor = Color.White
            End If
            tb.appendtext(line)
        Loop Until linecount = 15
        updatefrm(tb)
    End Sub

Private Sub updatefrm(ByVal tb As RichTextBox)

      
        If InvokeRequired Then
            Invoke(Sub() FlowLayoutPanel1.Controls.Add(tb))
        Else : FlowLayoutPanel1.Controls.Add(tb)
        End If
    End Sub