[451048] Avoids IllegalArgumentException for custom diff types

Makes sure that DiffUtil.getTargetFeature and
DiffUtil.getTargetContainer is only called for supported diff types but
not for potentially added custom diff types.

Bug: 451048
Change-Id: I02bc4048ade54120864e2154aa5dbb146e02bc4f
Signed-off-by: Philip Langer <planger@eclipsesource.com>