BUG: do not add an observer on the source of the first input if it does not exist
BUG: do not add an observer on the source of the first input if it does not exist

orfeotoolbox/OTB,orfeotoolbox/OTB,orfeotoolbox/OTB,orfeotoolbox/OTB,orfeotoolbox/OTB,orfeotoolbox/OTB