NullPointerException using Bellman-Ford algorithm

[miguellopezbecona]

Executing the following code, using one of the graph of the examples, raises a NullPointerException when using Bellman-Ford algorithm:

/*******************/
String first = "A";
String goal = "F";
List list = new ArrayList<>();
list.add(new Link("A","B",2));
list.add(new Link("A","C",6));
list.add(new Link("B","C",1));
list.add(new Link("B","D",9));
list.add(new Link("B","F",11));
list.add(new Link("C","E",3));
list.add(new Link("D","F",2));
list.add(new Link("E","D",2));
HashBasedHipsterDirectedGraph g = new HashBasedHipsterDirectedGraph<>();
for(Link l : list)
g.connect(l.getSource(), l.getTarget(), l.getWeight());
SearchProblem p = GraphSearchProblem.startingFrom(first).in(g).takeCostsFromEdges().build();
Algorithm.SearchResult o = Hipster.createBellmanFord(p).search(goal);
/*******************/
Note: "Link" is a simple bean class with "source", "target" and "weight" fields.

Replacing the algorithm with other such as Dijkstra or Breadth, it works fine.