#vulnerable code 
                                public Expression parse(ParserData data, SourceStream source, ExpressionParserSettings settings) {
        ExpressionContext context = new ExpressionContext(this, data, source);
        ExpressionParserWorker worker = new ExpressionParserWorker(this, context, source, subparsers, settings.isCombined());

        for (TokenRepresentation representation : context.getDiffusedSource()) {
            if (!worker.next(context.withUpdatedToken(representation))) {
                break;
            }
        }
        worker.finish(context);

        // if something went wrong
        if (worker.hasError()) {
            throw new ExpressionParserException(worker.getError().getErrorMessage(), context, worker.getError().getSource());
        }

        // if context does not contain any results
        if (!context.hasResults()) {
            throw new ExpressionParserException("Unknown expression", context, source.toSnippet());
        }

        for (ExpressionResultProcessor processor : new ReversedIterable<>(context.getProcessors())) {
            ExpressionResult<Expression> result = processor.process(context, context.getResults());

            if (result.containsError()) {
                throw new ExpressionParserException("Error occurred while processing the result: ", result.getErrorMessage(), context, context.getSource().toSnippet());
            }

            context.getResults().push(result.get());
        }

        // if worker couldn't prepare the final result
        if (context.getResults().size() > 1) {
            throw new ExpressionParserException("Source contains " + context.getResults().size() + " expressions", context, source.toSnippet());
        }

        source.read(worker.getLastSucceededRead());
        return context.getResults().pop();
    } 
                            #location 23 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public Expression parse(ParserData data, SourceStream source, ExpressionParserSettings settings) {
        return parse(data, source, settings, null);
    }