c# - NullReference exception when using AddPrecalculatedGraph

Question

i need to to some work with Edge object before adding it to graph, so i create it like this

private void AddNewEdgeToGraph(string sourceNodeID, string targetNodeID)
    {
        Edge newEdge = new Edge(sourceNodeID, "", targetNodeID);
        newEdge.LabelText = newEdge.Attr.Weight.ToString();
        graph.AddPrecalculatedEdge(newEdge);
    }

And, as you have guessed, i get the NullReferenceException on the last line of code. I doubleChecked the parameters for correctness, and i am sure the graph contains nodes with such IDs. I have also tried to add Edges this way:

Edge newEdge = new Edge(graph.FindNode("path1"), graph.FindNode("path2"), 
     ConnectionToGraph.Connected);
newEdge.LabelText = newEdge.Attr.Weight.ToString();
graph.AddPrecalculatedEdge(newEdge);

And this one works correctly. I thought that the first variant uses the second underneath the hood, so what may be the problem?