network: Small fix.

src/view/network/GraphWidget.py

@@ -596,7 +596,7 @@ class GraphWidget(QGraphicsView):
             self.mouse_origin_y = pos.y()
 
             items = self.items(event.pos())
-            if type(items[0]) == EdgeItem:
+            if items and type(items[0]) == EdgeItem:
                 edge = items[0]
                 if edge:
                     self.setCurrentEdge(edge)