GanttChart TypeError: content is null

Gantt
#1

Hello,
I am trying to start GanttChart v1.3 but without success.
I am using example from docs.dhtmlx.com/doku.php?id=dhtm … ding_tasks
and FireFox 19.0.2
When I am starting my page it is empty and i have one error which states:
TypeError: content is null dhtmlxgantt.js Line 183

My page below:

<script>
    var gantt = new GanttChart();

    var project_1 = new GanttProjectInfo(1, "Applet redesign", new Date(2006, 5, 11));
    var task_1 = new GanttTaskInfo(1, "Old code review", new Date(2006, 5, 11), 208, 50, "");
    task_1.addChildTask(new GanttTaskInfo(2, "Convert to J#", new Date(2006, 5, 11), 100, 40, ""));
    project_1.addTask(task_1);
    gantt.addProject(project_1);

    gantt.create("GanttDiv");
</script>

Please help,
Thanks in Advance
Yarko