[solved] how to show sub datasheet on form?

[asmdev]

hi,
i wish to load a sub datasheet on form based on a value of a textbox (in that form also)

i use the following code
the sPunchCard is a textbox which contained the value of the wanna loaded table name.

Private Sub Form_Current()
    
    If Len(sPunchCard) < 2 Then
        Exit Sub
    Else
        If table_exists(sPunchCard) = True Then
            SSubForm.Visible = True
            SSubForm.Locked = True
            SSubForm.SourceObject = "TABLE." & sPunchCard
        Else
            SSubForm.Visible = False
        End If
    End If
End Sub
 
Public Function table_exists(tblName As String) As Boolean
Dim i As Integer
    table_exists = False
    
    For i = 0 To CurrentDb.TableDefs.Count - 1
        If CurrentDb.TableDefs(i).Name = tblName Then
            table_exists = True
            Exit Function
        End If
    Next
End Function

the code functions well, but when it comes to the last record, and after use press the next record, it presents with an error box

Run-time error '94':
Invalid use of Null

anyone have any ideas?