#vulnerable code 
                                public Expression parse(ParserInfo info, TokenizedSource expressionSource) {
        if (expressionSource.size() == 1) {
            Token token = expressionSource.getToken(0);
            String value = token.getTokenValue();

            if (token.getType() == TokenType.LITERAL) {
                switch (token.getTokenValue()) {
                    case "null":
                        return new Expression(new PandaValue(null, null));
                    case "true":
                        return toSimpleKnownExpression("panda.lang:Boolean", true);
                    case "false":
                        return toSimpleKnownExpression("panda.lang:Boolean", false);
                    case "this":
                        ClassPrototype type = info.getComponent(Components.CLASS_PROTOTYPE);
                        return new Expression(type, new ThisExpressionCallback());
                    default:
                        throw new PandaParserException("Unknown literal: " + token);
                }
            }

            if (token.getType() == TokenType.SEQUENCE) {
                switch (token.getName()) {
                    case "String":
                        return toSimpleKnownExpression("panda.lang:String", value);
                    default:
                        throw new PandaParserException("Unknown sequence: " + token);
                }
            }

            if (NumberUtils.isNumber(value)) {
                return toSimpleKnownExpression("panda.lang:Int", Integer.parseInt(value));
            }

            ScopeLinker scopeLinker = info.getComponent(Components.SCOPE_LINKER);
            Scope scope = scopeLinker.getCurrentScope();
            Variable variable = VariableParserUtils.getVariable(scope, value);

            if (variable != null) {
                int memoryIndex = VariableParserUtils.indexOf(scope, variable);
                return new Expression(variable.getType(), new VariableExpressionCallback(memoryIndex));
            }

            ClassPrototype prototype = info.getComponent(Components.CLASS_PROTOTYPE);
            Field field = prototype.getField(value);

            if (field != null) {
                int memoryIndex = prototype.getFields().indexOf(field);
                return new Expression(field.getType(), new FieldExpressionCallback(field, memoryIndex));
            }
        }
        else if (TokenUtils.equals(expressionSource.get(0), TokenType.KEYWORD, "new")) {
            InstanceExpressionParser callbackParser = new InstanceExpressionParser();

            callbackParser.parse(expressionSource, info);
            InstanceExpressionCallback callback = callbackParser.toCallback();

            return new Expression(callback.getReturnType(), callback);
        }

        PandaTokenReader expressionReader = new PandaTokenReader(expressionSource);
        List<TokenizedSource> methodMatches = MethodInvokerExpressionParser.PATTERN.match(expressionReader);

        if (methodMatches != null && methodMatches.size() > 0) {
            MethodInvokerExpressionParser callbackParser = new MethodInvokerExpressionParser(methodMatches);

            callbackParser.parse(expressionSource, info);
            MethodInvokerExpressionCallback callback = callbackParser.toCallback();

            return new Expression(callback.getReturnType(), callback);
        }

        List<TokenizedSource> fieldMatches = FieldParser.PATTERN.match(expressionReader);

        if (fieldMatches != null && fieldMatches.size() > 0) {

        }

        throw new PandaParserException("Cannot recognize expression: " + expressionSource.toString());
    } 
                            #location 58 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public Expression parse(ParserInfo info, TokenizedSource expressionSource) {
        if (expressionSource.size() == 1) {
            Token token = expressionSource.getToken(0);
            String value = token.getTokenValue();

            if (token.getType() == TokenType.LITERAL) {
                switch (token.getTokenValue()) {
                    case "null":
                        return new Expression(new PandaValue(null, null));
                    case "true":
                        return toSimpleKnownExpression("panda.lang:Boolean", true);
                    case "false":
                        return toSimpleKnownExpression("panda.lang:Boolean", false);
                    case "this":
                        ClassPrototype type = info.getComponent(Components.CLASS_PROTOTYPE);
                        return new Expression(type, new ThisExpressionCallback());
                    default:
                        throw new PandaParserException("Unknown literal: " + token);
                }
            }

            if (token.getType() == TokenType.SEQUENCE) {
                switch (token.getName()) {
                    case "String":
                        return toSimpleKnownExpression("panda.lang:String", value);
                    default:
                        throw new PandaParserException("Unknown sequence: " + token);
                }
            }

            if (NumberUtils.isNumber(value)) {
                return toSimpleKnownExpression("panda.lang:Int", Integer.parseInt(value));
            }

            ScopeLinker scopeLinker = info.getComponent(Components.SCOPE_LINKER);
            Scope scope = scopeLinker.getCurrentScope();
            Variable variable = VariableParserUtils.getVariable(scope, value);

            if (variable != null) {
                int memoryIndex = VariableParserUtils.indexOf(scope, variable);
                return new Expression(variable.getType(), new VariableExpressionCallback(memoryIndex));
            }

            ClassPrototype prototype = info.getComponent(Components.CLASS_PROTOTYPE);
            Field field = prototype.getField(value);

            if (field != null) {
                int memoryIndex = prototype.getFields().indexOf(field);
                return new Expression(field.getType(), new FieldExpressionCallback(ThisExpressionCallback.asExpression(prototype), field, memoryIndex));
            }
        }
        else if (TokenUtils.equals(expressionSource.get(0), TokenType.KEYWORD, "new")) {
            InstanceExpressionParser callbackParser = new InstanceExpressionParser();

            callbackParser.parse(expressionSource, info);
            InstanceExpressionCallback callback = callbackParser.toCallback();

            return new Expression(callback.getReturnType(), callback);
        }

        PandaTokenReader expressionReader = new PandaTokenReader(expressionSource);
        List<TokenizedSource> methodMatches = MethodInvokerExpressionParser.PATTERN.match(expressionReader);

        if (methodMatches != null && methodMatches.size() > 0) {
            MethodInvokerExpressionParser callbackParser = new MethodInvokerExpressionParser(methodMatches);

            callbackParser.parse(expressionSource, info);
            MethodInvokerExpressionCallback callback = callbackParser.toCallback();

            return new Expression(callback.getReturnType(), callback);
        }

        List<TokenizedSource> fieldMatches = FIELD_PATTERN.match(expressionReader);

        if (fieldMatches != null && fieldMatches.size() == 2) {
            Expression instanceExpression = parse(info, fieldMatches.get(0));
            ClassPrototype instanceType = instanceExpression.getReturnType();
            Field instanceField = instanceType.getField(fieldMatches.get(1).getLast().getToken().getTokenValue());

            if (instanceField == null) {

            }

            int memoryIndex = instanceType.getFields().indexOf(instanceField);
            return new Expression(instanceType, new FieldExpressionCallback(instanceExpression, instanceField, memoryIndex));
        }

        throw new PandaParserException("Cannot recognize expression: " + expressionSource.toString());
    }