Set a better(?) fallback for unusable BIRD.

poor_mans_visualisation.py

@@ -12,10 +12,22 @@ if len(sys.argv) > 1:
 
 ref_topo = OspfFileTopologyProvider(ref_topo_file).get_topology()
 
-try:
-	cur_topo = BirdSocketTopologyProvider(instance='ospf1', area=0).get_topology()
-except OSError as e:
-	raise NotImplementedError('Cannot create a mock topology atm') from e
+def get_empty_topology():
+	from birdvisu.topo_v3 import TopologyV3
+	topo = TopologyV3()
+	topo.freeze()
+	return topo
+
+if len(sys.argv) > 2 and sys.argv[2] == '--no-bird':
+	cur_topo = get_empty_topology()
+else:
+	try:
+		cur_topo = BirdSocketTopologyProvider(instance='ospf1', area=0).get_topology()
+	except OSError as e:
+		# I know this is not how you print exceptions.
+		print('Cannot get topology from BIRD: {e}')
+		print('Will provide an empty topology.')
+		cur_topo = get_empty_topology()
 
 # Create combined topology