Some defensive programming

Apparently, pluginService#createInstance can return null... ;-)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
PGM format: only add channel axis for color images

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore temporary files written by vi

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>