PickFirstSelectionSet Runtime errors

Many of you are likely familiar with this problem and I have been
trying to figure out a solution but haven't come up with anything.

I have code that is generating a runtime error whenever the
PickFirstSelectionSet is accessed after the user tries to open a file
but cancels out of it.

Private Sub AcadDocument_BeginRightClick(ByVal PickPoint As Variant)

'Finds what's in the selection set we just right-clicked on
GetPickfirstSelectionSet
End Sub


Public Function GetPickfirstSelectionSet()
' This loops through all the objects in the pickfirst selection
set.

Dim pfSS As AcadSelectionSet
Dim ssobject As AcadEntity
Dim vAtts

Set pfSS = ThisDrawing.PickfirstSelectionSet ' RUNTIME ERROR
HERE


For Each ssobject In pfSS
If TypeOf ssobject Is AcadBlockReference Then
If Left$(ssobject.Name, 5) = "ESCD_" Then
Set ThisDrawing.oBlock = ssobject
vAtts = ssobject.GetAttributes
For ix = LBound(vAtts) To UBound(vAtts) Step 1
If vAtts(ix).TagString = "BLOCK_ID" Then
If Not vAtts(ix).TextString = "" Then
ThisDrawing.iBlockID =
CInt(vAtts(ix).TextString)
End If
End If
Next ix

End If
End If
Next ssobject

EnableMenus 'enable appropriate menus
GetPickfirstSelectionSet = ThisDrawing.bBlock

End Function



So this all works fine until the user tries to open a file but then
cancels out of it. Then the line ThisDrawing.PickFirstSelectionSet
creates a Runtime Error with the error:

Calling method AddItems of interface IAcadSelectionSet failed

Since the PickFirstSelectionSet is read only I cannot manipulate it.
Any idea what can cause this and how to fix/workaround it if its an
Autocad bug?
Many thanks,
Paul