setting current pviewport

Hi,
This is connected to a simple form consisting of an ok & cancel button and a listbox of layouts. I'm trying to be able to select layouts from the listbox and freeze the last layer created in all pviewports of the layouts selected. It seems to almost work, but I get an 'error setting current viewport' that I've been having trouble figuring out. Any ideas? thanks.

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Private Sub ok_Click()
Dim i As Integer, x As Integer, y As Integer
For i = 0 To ThisDrawing.Layouts.count - 1
    If LayoutListBox.Selected(i) = True Then
    ThisDrawing.ActiveLayout = ThisDrawing.Layouts.Item(i)
    Dim mypViewport As AcadPViewport
    Dim myObj As AcadObject
    Dim pvCount As Integer
    ThisDrawing.ActiveSpace = acPaperSpace



For x = 0 To ThisDrawing.PaperSpace.count - 1
        Set myObj = ThisDrawing.PaperSpace.Item(x)
        If myObj.EntityName = "AcDbViewport" Then
            Set mypViewport = ThisDrawing.PaperSpace.Item(x)
            mypViewport.Display (True)
            ThisDrawing.mspace = True
            ThisDrawing.ActivePViewport = mypViewport '?
            ThisDrawing.SendCommand "VPLAYER" & vbCr & "F" & vbCr & ThisDrawing.LAYERS.Item(ThisDrawing.LAYERS.count - 1).Name & vbCr & vbCr & vbCr
            ThisDrawing.mspace = False
       End If
    Next x
End If
Next i
End
End Sub

Private Sub UserForm_Initialize()
LayoutForm.Caption = "VPLayer Freeze For Layouts - For Layer: " & ThisDrawing.LAYERS.Item(ThisDrawing.LAYERS.count - 1).Name
Dim x As Integer

For x = 0 To ThisDrawing.Layouts.count - 1
    LayoutListBox.AddItem ThisDrawing.Layouts.Item(x).Name
Next x
End Sub
Private Sub cancelbutton_Click()
End
End Sub
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