#fixed code
                              private UndiGraph<Var, Void> computeVariableNamesInterferenceGraph(
      NodeTraversal t, ControlFlowGraph<Node> cfg, Set<Var> escaped) {
    UndiGraph<Var, Void> interferenceGraph =
        LinkedUndirectedGraph.create();

    // For all variables V not in unsafeCrossRange,
    // LiveRangeChecker(V, X) and LiveRangeChecker(Y, V) will never add a edge
    // to the interferenceGraph. In other words, we don't need to use
    // LiveRangeChecker on variable pair (A, B) if both A and B are not
    // in the unsafeCrossRangeSet. See PrescreenCrossLiveRange for details.
    Set<Var> unsafeCrossRangeSet = Sets.newHashSet();
    Scope scope = t.getScope();

    for (DiGraphNode<Node, Branch> cfgNode : cfg.getDirectedGraphNodes()) {
      if (cfg.isImplicitReturn(cfgNode)) {
        continue;
      }

      for (Iterator<Var> i = scope.getVars(); i.hasNext();) {
        final Var v = i.next();

        if (!unsafeCrossRangeSet.contains(v)) {
          FlowState<LiveVariableLattice> state = cfgNode.getAnnotation();
          PrescreenCrossLiveRange check =
              new PrescreenCrossLiveRange(v, state.getOut());

          NodeTraversal.traverse(compiler, cfgNode.getValue(), check);

          if (!check.isSafe()) {
            unsafeCrossRangeSet.add(v);
          }
        }
      }
    }

    // First create a node for each non-escaped variable.
    for (Iterator<Var> i = scope.getVars(); i.hasNext();) {
      Var v = i.next();
      if (!escaped.contains(v)) {

        // TODO(user): In theory, we CAN coalesce function names just like
        // any variables. Our Liveness analysis captures this just like it as
        // described in the specification. However, we saw some zipped and
        // and unzipped size increase after this. We are not totally sure why
        // that is but, for now, we will respect the dead functions and not play
        // around with it.
        if (!NodeUtil.isFunction(v.getParentNode())) {
          interferenceGraph.createNode(v);
        }
      }
    }

    // Go through each variable and try to connect them.
    for (Iterator<Var> i1 = scope.getVars(); i1.hasNext();) {
      Var v1 = i1.next();

      NEXT_VAR_PAIR:
      for (Iterator<Var> i2 = scope.getVars(); i2.hasNext();) {
        Var v2 = i2.next();

        // Skip duplicate pairs.
        if (v1.index >= v2.index) {
          continue;
        }

        if (!interferenceGraph.hasNode(v1) ||
            !interferenceGraph.hasNode(v2)) {
          // Skip nodes that were not added. They are globals and escaped
          // locals. Also avoid merging a variable with itself.
          continue NEXT_VAR_PAIR;
        }

        if (v1.getParentNode().getType() == Token.LP &&
            v2.getParentNode().getType() == Token.LP) {
          interferenceGraph.connectIfNotFound(v1, null, v2);
          continue NEXT_VAR_PAIR;
        }

        // Go through every CFG node in the program and look at
        // this variable pair. If they are both live at the same
        // time, add an edge between them and continue to the next pair.
        NEXT_CROSS_CFG_NODE:
        for (DiGraphNode<Node, Branch> cfgNode : cfg.getDirectedGraphNodes()) {
          if (cfg.isImplicitReturn(cfgNode)) {
            continue NEXT_CROSS_CFG_NODE;
          }

          FlowState<LiveVariableLattice> state = cfgNode.getAnnotation();
          // Check the live states and add edge when possible.
          if ((state.getIn().isLive(v1) && state.getIn().isLive(v2)) ||
              (state.getOut().isLive(v1) && state.getOut().isLive(v2))) {
            interferenceGraph.connectIfNotFound(v1, null, v2);
            continue NEXT_VAR_PAIR;
          }
        }

        // v1 and v2 might not have an edge between them! woohoo. there's
        // one last sanity check that we have to do: we have to check
        // if there's a collision *within* the cfg node.
        if (!unsafeCrossRangeSet.contains(v1) &&
            !unsafeCrossRangeSet.contains(v2)) {
          continue NEXT_VAR_PAIR;
        }

        NEXT_INTRA_CFG_NODE:
        for (DiGraphNode<Node, Branch> cfgNode : cfg.getDirectedGraphNodes()) {
          if (cfg.isImplicitReturn(cfgNode)) {
            continue NEXT_INTRA_CFG_NODE;
          }
          FlowState<LiveVariableLattice> state = cfgNode.getAnnotation();
          boolean v1OutLive = state.getOut().isLive(v1);
          boolean v2OutLive = state.getOut().isLive(v2);
          CombinedLiveRangeChecker checker = new CombinedLiveRangeChecker(
              new LiveRangeChecker(v1, v2OutLive ? null : v2),
              new LiveRangeChecker(v2, v1OutLive ? null : v1));
          NodeTraversal.traverse(
              compiler,
              cfgNode.getValue(),
              checker);
          if (checker.connectIfCrossed(interferenceGraph)) {
            continue NEXT_VAR_PAIR;
          }
        }
      }
    }
    return interferenceGraph;
  }
#vulnerable code 
                              private UndiGraph<Var, Void> computeVariableNamesInterferenceGraph(
      NodeTraversal t, ControlFlowGraph<Node> cfg, Set<Var> escaped) {
    UndiGraph<Var, Void> interferenceGraph =
        LinkedUndirectedGraph.create();
    Scope scope = t.getScope();

    // First create a node for each non-escaped variable.
    for (Iterator<Var> i = scope.getVars(); i.hasNext();) {
      Var v = i.next();
      if (!escaped.contains(v)) {

        // TODO(user): In theory, we CAN coalesce function names just like
        // any variables. Our Liveness analysis captures this just like it as
        // described in the specification. However, we saw some zipped and
        // and unzipped size increase after this. We are not totally sure why
        // that is but, for now, we will respect the dead functions and not play
        // around with it.
        if (!NodeUtil.isFunction(v.getParentNode())) {
          interferenceGraph.createNode(v);
        }
      }
    }

    // Go through each variable and try to connect them.
    for (Iterator<Var> i1 = scope.getVars(); i1.hasNext();) {
      Var v1 = i1.next();

      NEXT_VAR_PAIR:
      for (Iterator<Var> i2 = scope.getVars(); i2.hasNext();) {
        Var v2 = i2.next();

        // Skip duplicate pairs.
        if (v1.index >= v2.index) {
          continue;
        }

        if (!interferenceGraph.hasNode(v1) ||
            !interferenceGraph.hasNode(v2)) {
          // Skip nodes that were not added. They are globals and escaped
          // locals. Also avoid merging a variable with itself.
          continue NEXT_VAR_PAIR;
        }

        if (v1.getParentNode().getType() == Token.LP &&
            v2.getParentNode().getType() == Token.LP) {
          interferenceGraph.connectIfNotFound(v1, null, v2);
          continue NEXT_VAR_PAIR;
        }

        // Go through every CFG node in the program and look at
        // this variable pair. If they are both live at the same
        // time, add an edge between them and continue to the next pair.
        NEXT_CROSS_CFG_NODE:
        for (DiGraphNode<Node, Branch> cfgNode : cfg.getDirectedGraphNodes()) {
          if (cfg.isImplicitReturn(cfgNode)) {
            continue NEXT_CROSS_CFG_NODE;
          }

          FlowState<LiveVariableLattice> state = cfgNode.getAnnotation();
          // Check the live states and add edge when possible.
          if ((state.getIn().isLive(v1) && state.getIn().isLive(v2)) ||
              (state.getOut().isLive(v1) && state.getOut().isLive(v2))) {
            interferenceGraph.connectIfNotFound(v1, null, v2);
            continue NEXT_VAR_PAIR;
          }
        }

        // v1 and v2 might not have an edge between them! woohoo. there's
        // one last sanity check that we have to do: we have to check
        // if there's a collision *within* the cfg node.
        NEXT_INTRA_CFG_NODE:
        for (DiGraphNode<Node, Branch> cfgNode : cfg.getDirectedGraphNodes()) {
          if (cfg.isImplicitReturn(cfgNode)) {
            continue NEXT_INTRA_CFG_NODE;
          }

          FlowState<LiveVariableLattice> state = cfgNode.getAnnotation();
          boolean v1OutLive = state.getOut().isLive(v1);
          boolean v2OutLive = state.getOut().isLive(v2);
          CombinedLiveRangeChecker checker = new CombinedLiveRangeChecker(
              new LiveRangeChecker(v1, v2OutLive ? null : v2),
              new LiveRangeChecker(v2, v1OutLive ? null : v1));
          NodeTraversal.traverse(
              compiler,
              cfgNode.getValue(),
              checker);
          if (checker.connectIfCrossed(interferenceGraph)) {
            continue NEXT_VAR_PAIR;
          }
        }
      }
    }
    return interferenceGraph;
  } 
                            #location 18 
                            #vulnerability type NULL_DEREFERENCE