output: #fixed code
                                private void verifyNode(Node node) {
      if (node == null)
        return;
  
      // Pre-order traversal.
      verifyNodes(node.children());
  
      if (node instanceof Call) {
        Call call = (Call) node;
  
        // Skip resolving property derefs.
        if (call.isJavaStatic() || call.isPostfix())
          return;
  
        verifyNode(call.args());
  
        FunctionDecl targetFunction = resolveCall(call.name);
        if (targetFunction == null)
          addError("Cannot resolve function: " + call.name, call.sourceLine, call.sourceColumn);
        else {
          // Check that the args are correct.
          int targetArgs = targetFunction.arguments().children().size();
          int calledArgs = call.args().children().size();
          if (calledArgs != targetArgs)
            addError("Incorrect number of arguments to: " + targetFunction.name()
                + " (expected " + targetArgs + ", found "
                + calledArgs + ")",
                call.sourceLine, call.sourceColumn);
        }
  
      } else if (node instanceof PatternRule) {
        PatternRule patternRule = (PatternRule) node;
        verifyNode(patternRule.rhs);
  
        // Some sanity checking of pattern rules.
        FunctionDecl function = functionStack.peek().function;
        int argsSize = function.arguments().children().size();
        int patternsSize = patternRule.patterns.size();
        if (patternsSize != argsSize)
          addError("Incorrect number of patterns in: '" + function.name() + "' (expected " + argsSize
              + " found " + patternsSize + ")", patternRule.sourceLine, patternRule.sourceColumn);
  
      } else if (node instanceof Guard) {
        Guard guard = (Guard) node;
        verifyNode(guard.expression);
        verifyNode(guard.line);
      } else if (node instanceof Variable) {
        Variable var = (Variable) node;
        if (!resolveVar(var.name))
          addError("Cannot resolve symbol: " + var.name, var.sourceLine, var.sourceColumn);
      } else if (node instanceof ConstructorCall) {
        ConstructorCall call = (ConstructorCall) node;
        if (!resolveType(call))
          addError("Cannot resolve type (either as loop or Java): "
              + (call.modulePart == null ? "" : call.modulePart) + call.name,
              call.sourceLine, call.sourceColumn);
      } else if (node instanceof Assignment) {
        // Make sure that you cannot reassign function arguments.
        Assignment assignment = (Assignment) node;
        if (assignment.lhs() instanceof Variable) {
          Variable lhs = (Variable) assignment.lhs();
  
          FunctionContext functionContext = functionStack.peek();
          for (Node argument : functionContext.function.arguments().children()) {
            ArgDeclList.Argument arg = (ArgDeclList.Argument) argument;
            if (arg.name().equals(lhs.name))
              addError("Illegal argument reassignment (declare a local variable instead)",
                  lhs.sourceLine, lhs.sourceColumn);
          }
  
  //        verifyNode(assignment.rhs());
        }
      }
    }
input: #vulnerable code 
                                private void verifyNode(Node node) {
      if (node == null)
        return;
  
      // Pre-order traversal.
      verifyNodes(node.children());
  
      if (node instanceof Call) {
        Call call = (Call) node;
  
        // Skip resolving property derefs.
        if (call.isJavaStatic() || call.isPostfix())
          return;
  
        verifyNode(call.args());
  
        FunctionDecl targetFunction = resolveCall(call.name);
        if (targetFunction == null)
          addError("Cannot resolve function: " + call.name, call.sourceLine, call.sourceColumn);
        else {
          // Check that the args are correct.
          int targetArgs = targetFunction.arguments().children().size();
          int calledArgs = call.args().children().size();
          if (calledArgs != targetArgs)
            addError("Incorrect number of arguments to: " + targetFunction.name()
                + " (expected " + targetArgs + ", found "
                + calledArgs + ")",
                call.sourceLine, call.sourceColumn);
        }
  
      } else if (node instanceof PatternRule) {
        PatternRule patternRule = (PatternRule) node;
        verifyNode(patternRule.rhs);
  
        // Some sanity checking of pattern rules.
        FunctionDecl function = functionStack.peek().function;
        int argsSize = function.arguments().children().size();
        int patternsSize = patternRule.patterns.size();
        if (patternsSize != argsSize)
          addError("Incorrect number of patterns in: '" + function.name() + "' (expected " + argsSize
              + " found " + patternsSize + ")", patternRule.sourceLine, patternRule.sourceColumn);
  
      } else if (node instanceof Guard) {
        Guard guard = (Guard) node;
        verifyNode(guard.expression);
        verifyNode(guard.line);
      } else if (node instanceof Variable) {
        Variable var = (Variable) node;
        if (!resolveVar(var.name))
          addError("Cannot resolve symbol: " + var.name, var.sourceLine, var.sourceColumn);
      } else if (node instanceof ConstructorCall) {
        ConstructorCall call = (ConstructorCall) node;
        if (!resolveType(call))
          addError("Cannot resolve type (either as loop or Java): "
              + (call.modulePart == null ? "" : call.modulePart) + call.name,
              call.sourceLine, call.sourceColumn);
      }
    } 
                              #location 23 
                              #vulnerability type NULL_DEREFERENCE