Fix .equals for Node.

src/main/org/insa/graph/Node.java

@@ -95,6 +95,9 @@ public class Node implements Comparable<Node> {
 
     @Override
     public boolean equals(Object other) {
+        if (other == null) {
+            return false;
+        }
         if (other instanceof Node) {
             return getId() == ((Node) other).getId();
         }