[442761] Fix regression due to commit 1077eea

Fix ArrangeBorderNodesAction#getCommand(). Check that the diagram
graphical viewer is valid to avoid NPE. This check was also done in
SelectHiddenElementsAction and SelectPinnedElementsAction

Bug: 442761
Change-Id: Ibc818ed6258226673cedad17940304c37f4c2737
Signed-off-by: Mickael LANOE <mickael.lanoe@obeo.fr>