Do not crash if cannot find icon

sugar/session/session.py

@@ -9,7 +9,7 @@ from sugar.shell import shell
 
 def start(console):
 	shell.main()
-	print 'aaaa'
+
 	activities = ['sugar/chat/chat', 'sugar/browser/browser']
 
 	for activity in activities:

sugar/shell/activity.py

@@ -105,6 +105,7 @@ class Activity(dbus.service.Object):
 	def activity_set_tab_icon_name(self, icon_name):
 		icon_theme = gtk.icon_theme_get_default()
 		icon_info = icon_theme.lookup_icon(icon_name, gtk.ICON_SIZE_MENU, 0)
+		if icon_info:
 			pixbuf = icon_info.load_icon()
 			scaled_pixbuf = pixbuf.scale_simple(16, 16, gtk.gdk.INTERP_BILINEAR)
 			self.activity_set_icon(scaled_pixbuf)