Uncaught TypeError: Cannot read property 'data_types' of null

    var _chart;
    var _testIndex = 0;
    function test() {
        _testIndex++;
        console.info("testing "+_testIndex);
        if (_chart != null) {
            _chart.destroy();
            _chart = null;
        }
        var x = {
            data: { columns: [["x"], ["y"]], x: "x", }
        };
        for (var i = 1; i < 300; i++) {
            x.data.columns[0].push(i * 2);
            x.data.columns[1].push(i * 3);
        }
        _chart = c3.generate(x);
        if(_testIndex<20)
            scheduleTest();
    }

    test();
    function scheduleTest() {
        window.setTimeout(test, 1000);
    }