treat each target's root node like a start if nullptr is given as start node
treat each target's root node like a start if nullptr is given as start node

dartsim/dart,axeisghost/DART6motionBlur,dartsim/dart,axeisghost/DART6motionBlur,axeisghost/DART6motionBlur,dartsim/dart,dartsim/dart,dartsim/dart