Bug 447942 - "Widget is disposed" Exception in ViewportLayer

Reset the layer's scrollbar handlers to null when the layer has been
disposed. This prevents the layer from accessing the table's scrollbars
in response to any IStructuralChangeEvent being handled afterwards.

Change-Id: Idd42384e5d52fcbc83da9d50159152fa725f43dc
Signed-off-by: mosebach@patronas.de <mosebach@patronas.de>