HTML5 File System API error "Cannot read property 'root' of undefined"

Question

When I try to write a file I am getting this error in Google Chrome:

Uncaught TypeError: Cannot read property 'root' of undefined FileSystemApi.html:29
onButtonSaveClick FileSystemApi.html:29
onclick

My sample code is as follows

var fileSys;

$(function () {
    initFileSystem(onInitFileSystemSuccess);
});

function onInitFileSystemSuccess(fs) {
    console.log('Opened file system: ' + fs.name);
    fileSys = fs;
}

function onButtonSaveClick() {
    alert('1');
    var fileName = $("#fileName").val();
    alert(fileName);
    var fileContent = $("#fileContent").val();
    alert(fileContent);
    alert(fileSys.root);
    fileSys.root.getFile(fileName, {
            create: true,
            exclusive: true
        },
        function (fileEntry) {
            alert('2');
            console.log('File opened: ' + fileEntry.fullPath);
            fileEntry.createWriter(function (fileWriter) {
                var blob = new Blob([fileContent], {
                    type: 'text/plain'
                });
                fileWriter.write(blob);
                alert('done');
            }, onFileSystemError);
        }, onFileSystemError);
}

Would anyone be able to point out the problem(s), and, if so, contribute a solution?