syntaxes/c++.json did not work with loadGrammar method

Problem: "c++.json" file in ".\test-cases\themes\syntaxes" can not be parsed to a grammar, using the way the readme.md file says.

I called loadGrammar method to parse this grammar file, But the callback's parameter was null. First , I thought the grammar file is wrong, so I try to test it with inspect.js. It was totally fine!
Then I used the same code in inspect.js, and it worked too. Not sure if this is a bug, But the readme.md file really need to check.

this is my code according to readme.md.

var vsctm = require("vscode-textmate");
var fs = require("fs");

let grammarPaths = {
    "source.js": "./config/grammars/JavaScript.tmLanguage.json",
    "source.c++":"./config/grammars/c++.json",
};

let registry = new vsctm.Registry({
    loadGrammar: function (scopeName) {
        let path = grammarPaths[scopeName];
        if (path) {
            return new Promise((c,e) => {
                let content = fs.readFileSync(path).toString();
                let grammar = vsctm.parseRawGrammar(content,path);
                console.log("RawGrammar is null",grammar==null);
                c(grammar);
            });
        }	
    }
});

function test(){
    registry.loadGrammar("source.js").then(grammar => {
        // grammar is not null
        console.log("source.js",grammar);
    });
    registry.loadGrammar("source.c++").then(grammar => {
        // grammar is null , I do not know why...
        console.log("source.c++",grammar);
    });
}

test()

this is the code in inspect.js

var GRAMMAR_PATHS = ["E:\\codeparser\\vscode-textmate-master\\test-cases\\themes\\syntaxes\\c++.json"]
var FILE_PATH = "E:\\codeparser\\vscode-textmate-master\\benchmark\\xxx.cpp"

// process.env["VSCODE_TEXTMATE_DEBUG"] = true;

var main = require("vscode-textmate");
var fs = require("fs");
var Registry = main.Registry;
var registry = new Registry();
var grammarPromise = null;

for (let path of GRAMMAR_PATHS) {
	var content = fs.readFileSync(path).toString();
	var rawGrammar = main.parseRawGrammar(content, path);
	var g = registry.addGrammar(rawGrammar);
	grammarPromise = grammarPromise || g;
}
grammarPromise.then(grammar => {
	console.log("source.c++",grammar)// grammar is not null  !
}

I am sure the I used the same "c++.json" file .

Reproduce the reported difference between Registry.loadGrammar and the inspect.js path using test-cases/themes/syntaxes/c++.json. Compare the README example with inspect.js and trace the loadGrammar callback for source.c++; the work is done when the cause of the null grammar is identified and the README or behavior is corrected and verified.