Fix dfki-ric#363 - 2

Author: AlpenAalAlex

phobos/core/multiple.py

@@ -19,7 +19,8 @@ def __init__(self, name=None, world=None, model=None, file=None, origin=None, fr
         assert world is not None
         self.model = _singular(model)
         self.origin = _singular(origin) if origin is not None else representation.Pose()
-        self._file = os.path.normpath(os.path.join(os.path.dirname(world.inputfile), file)) if not os.path.isabs(file) else file
+        self._file = os.path.normpath(os.path.join(os.path.dirname(world.inputfile), file)) \
+            if file is not None and not os.path.isabs(file) else file
         if model is None and file is not None:
             if self._file.lower().rsplit(".", 1)[-1] in ["smurfs", "smurfa"]:
                 try: