Fix GraphWidget.update() crashing when called without vprops or eprops argument.

This fixes interactive_window() crashing when called with window argument but without vprops or eprops.

src/graph_tool/draw/gtk_draw.py

@@ -20,6 +20,8 @@
 
 import numpy
 
+import copy
+
 from .. import GraphView, PropertyMap, ungroup_vector_property, \
     group_vector_property, infect_vertex_property, edge_endpoint_property, _prop
 from .cairo_draw import *
@@ -381,6 +383,9 @@ class GraphWidget(Gtk.DrawingArea):
                nodesfirst=None, display_props=None,
                fit_view=True, bg_color=None, **kwargs):
 
+        vprops = {} if vprops is None else copy.copy(vprops)
+        eprops = {} if eprops is None else copy.copy(eprops)
+
         props, kwargs = parse_props("vertex", kwargs)
         vprops.update(props)
         props, kwargs = parse_props("edge", kwargs)