[444728] Add test to ensure that there is no NPE on diagram deletion

Add a test to ensure that there is no NPE on diagram deletion when one
of the palette tools listens on it.

Bug: 444728
Change-Id: I97108c3d34570f0ece73e2175dd121e29ceb822d
Signed-off-by: Laurent Redor <laurent.redor@obeo.fr>