#vulnerable code 
                              private Node tryOptimizeSwitch(Node n) {
    Preconditions.checkState(n.getType() == Token.SWITCH);

    Node defaultCase = findDefaultCase(n);
    if (defaultCase != null && isUselessCase(defaultCase)) {
      NodeUtil.redeclareVarsInsideBranch(defaultCase);
      n.removeChild(defaultCase);
      reportCodeChange();
      defaultCase = null;
    }

    // Removing cases when there exists a default case is not safe.
    // TODO(johnlenz): Allow this if the same code is executed.
    if (defaultCase == null) {
      Node next = null;
      // The first child is the switch conditions skip it.
      for (Node c = n.getFirstChild().getNext(); c != null; c = next) {
        next = c.getNext();
        if (!mayHaveSideEffects(c.getFirstChild()) && isUselessCase(c)) {
          NodeUtil.redeclareVarsInsideBranch(c);
          n.removeChild(c);
          reportCodeChange();
        }
      }
    }

    if (n.hasOneChild()) {
      Node condition = n.removeFirstChild();
      Node parent = n.getParent();
      Node replacement = new Node(Token.EXPR_RESULT, condition)
                            .copyInformationFrom(n);
      parent.replaceChild(n, replacement);
      reportCodeChange();
      return replacement;
    }

    return null;
  } 
                            #location 17 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              private Node tryOptimizeSwitch(Node n) {
    Preconditions.checkState(n.getType() == Token.SWITCH);

    Node defaultCase = tryOptimizeDefaultCase(n);

    // Removing cases when there exists a default case is not safe.
    if (defaultCase == null) {
      Node next = null;
      Node prev = null;
      // The first child is the switch conditions skip it.
      for (Node c = n.getFirstChild().getNext(); c != null; c = next) {
        next = c.getNext();
        if (!mayHaveSideEffects(c.getFirstChild()) && isUselessCase(c, prev)) {
          removeCase(n, c);
        } else {
          prev = c;
        }
      }
    }

    // Remove the switch if there are no remaining cases.
    if (n.hasOneChild()) {
      Node condition = n.removeFirstChild();
      Node parent = n.getParent();
      Node replacement = new Node(Token.EXPR_RESULT, condition)
                            .copyInformationFrom(n);
      parent.replaceChild(n, replacement);
      reportCodeChange();
      return replacement;
    }

    return null;
  }