Fix bug when drawing empty graphs.

src/graph_tool/draw/cairo_draw.py

@@ -738,6 +738,8 @@ def get_bb(g, pos, size, pen_width, size_scale=1, text=None, font_family=None,
 
 def fit_to_view(g, pos, geometry, size, pen_width, M=None, text=None,
                 font_family=None, font_size=None, cr=None):
+    if g.num_vertices() == 0:
+        return [0, 0], 1
     if M is not None:
         pos_x, pos_y = ungroup_vector_property(pos, [0, 1])
         P = np.zeros((2, len(pos_x.fa)))

src/graph_tool/draw/gtk_draw.py

@@ -310,7 +310,7 @@ class GraphWidget(Gtk.DrawingArea):
 
     def layout_callback(self):
         """Perform one step of the layout algorithm."""
-        if self.layout_callback_id is None:
+        if self.layout_callback_id is None or self.g.num_vertices() == 0:
             return False
         pos_temp = ungroup_vector_property(self.pos, [0, 1])
         sfdp_layout(self.g, K=self.layout_K,