In TreeViewerFocusCellManager.getInitialFocusCell() the
tree.getTopItem() can return a null element, since the tree can be in
process (expanded, collapsed, scrolled or new items are added or
removed). So a null check must be added.

Change-Id: Icdc9b44b3ace1cc385e2a6e8cf43f04622c31020
Signed-off-by: Denis Ungemach <denis.ungemach@sap.com>