Fix NPE bug at startup.

The user could get a NPE when laucnhing a viewer early after launching MaMuT.
noted by Tobias:
It is easy to explain: in MaMuT.launch() you call
    launchGUI();
before
    prepareSources( file );
So the user has the opportunity to instantiate a viewer before the
  private List< SourceAndConverter< ? >> sources;
exists.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>