Referring to this question I asked: How to find the longest word in a trie?

I'm having trouble implementing the pseudocode given in the answer.

findLongest(trie):
 //first do a BFS and find the "last node"
 queue <- []
 queue.add(trie.root)
 last <- nil
 map <- empty map
while (not queue.empty()):
 curr <- queue.pop()
 for each son of curr:
    queue.add(son)
    map.put(son,curr) //marking curr as the parent of son
 last <- curr
//in here, last indicate the leaf of the longest word
//Now, go up the trie and find the actual path/string
curr <- last
str = ""
while (curr != nil):
      str = curr + str //we go from end to start   
    curr = map.get(curr)
return str

This is what I have for my method

public static String longestWord (DTN d) {
  Queue<DTN> holding = new ArrayQueue<DTN>();
  holding.add(d);
  DTN last = null;
  Map<DTN,DTN> test = new ArrayMap<DTN,DTN>();
  DTN curr;
  while (!holding.isEmpty()) {
       curr = holding.remove();

      for (Map.Entry<String, DTN> e : curr.children.entries()) {
          holding.add(curr.children.get(e));
          test.put(curr.children.get(e), curr);
      }
          last = curr;

      }
  curr = last;
  String str = "";
  while (curr != null) {
      str = curr + str;
      curr = test.get(curr);

  }
  return str;

  }

I'm getting a NullPointerException at:

 for (Map.Entry<String, DTN> e : curr.children.entries())

How can I find and fix the cause of the NullPointerException of the method so that it returns the longest word in a trie?