BUG-972: correct Precondition

This is just a tiny adjustment of a Precondition, which should have been
triggered before this access. isPresent() returns true/false, which will
be autoboxed, so will always pass checkNotNull. use checkArgument()
instead, which will ensure that isPresent() == true.

Change-Id: I53fe6913c7a0f8073a193bc76c0792059c26fe2a
Signed-off-by: Robert Varga <rovarga@cisco.com>