[FIX] NPE in ChangeColorManager

ERROR 2013-10-28 22:18:57,575 (Utils.java:325) Internal Error:
java.lang.NullPointerException

	at
de.fu_berlin.inf.dpp.project.internal.ChangeColorManager.updateColorSet(ChangeColorManager.java:585)
	at
de.fu_berlin.inf.dpp.project.internal.ChangeColorManager.reassignSessionColorIDs(ChangeColorManager.java:351)


All get*Users from the ISarosSession interface return snapshots that may
not always reflect the state of an activity provider that works with the
listener notification via userJoined and userLeft. As already discussed
in another patch set this methods needs a better description as they
should only use when the component that uses them is able to deal with
the returned results a.k.a lazy handling.

Change-Id: Ieae32b9e75e4c7cbbbf0d7834ac56057e11162e5
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1209
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>