Protected Overrides Sub WndProc(ByRef m As Message)
            If m.Msg = WM_LBUTTONUP Then
                MyBase.DefWndProc(m)
            End If
            MyBase.WndProc(m)
        End Sub

Private Shared Function GetGroupID(ByVal lstvwgrp As ListViewGroup) As _
  System.Nullable(Of Integer)
            Dim rtnval As System.Nullable(Of Integer) = Nothing
            Dim GrpTp As Type = lstvwgrp.[GetType]()
            If GrpTp IsNot Nothing Then
                Dim pi As PropertyInfo = GrpTp.GetProperty("ID", _
                  BindingFlags.NonPublic Or BindingFlags.Instance)
                If pi IsNot Nothing Then
                    Dim tmprtnval As Object = pi.GetValue(lstvwgrp, Nothing)
                    If tmprtnval IsNot Nothing Then
                        rtnval = TryCast(tmprtnval, System.Nullable(Of _
                          Integer)) ' <----- Fehler
                    End If
                End If
            End If
            Return rtnval
        End Function