How to add changed handler to grid?

[MaEcTPo]

I have some trouble.
My grid generates from datasource (XJSONDataSource)

TaskDS taskDS = TaskDS.getInstance();
grid.setDataSource(taskDS);

And I'd like to have depended selectboxes in it. I'm reading showcase examples and write my code.
So, I read http://www.smartclient.com/smartgwt/...t_selects_grid
and I was trying:

ListGridField[] fields = grid.getFields(); // for test
ListGridField departmentField = grid.getField("department");
department.addChangedHandler(new ChangedHandler() {
    @Override
    public void onChanged(ChangedEvent event) {
        grid.clearEditValue(event.getRowNum(), "assignedTo");
    }
});

A part of code from TaskDS:

DataSourceField assigneddepartment = new DataSourceField("department", FieldType.TEXT, "Department");

But departmentField and even fields is always null.

Where is my mistake?