xml: Replace a checkNotNull by a message when state undefined

An error in the XML where the next state defined by the user was undefined
would throw a NullPointerException and make the analysis fail ungracefully.
This logs a message to the user (we still need a way to have debug data on the
XML)

This is one step in solving bug 493155

Change-Id: I53edc74c6e9abcf531df07d17d7fa488c52e9155
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/72080
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>