Remove weird default and warning.

birdvisu/annotations/layout.py

@@ -49,12 +49,13 @@ class PlaceVerticesFromFile(StyleAnnotator):
 	The filename is either given to the Annotator, or ./visualisation.visu is used."""
 	idempotent = False # The layout file could have changed, this is not covered by a TopologyV3
 	def __init__(self, filename):
-		if filename is None:
-			filename = r'./visualisation.visu'
 		self.filename = filename
 	def annotate(self, topo):
 		self.topo = topo
 		self.result = Annotation()
+		if self.filename is None:
+			self.result.for_topology = False
+			return self.result
 		try:
 			with open(self.filename, 'r') as f:
 				positions = load(f)