TrackMate_: fix NPE when loading at the spot filtering state

If you loaded a TrackMate file saved when selecting spot filters,
you got a NPE, and the panel did not display correctly (needed manual
refresh).
This was because the spot filter component itself was not initialized in
this very specific case.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>