To reproduce:
- Enable the Trackball.
- Add 2 series to the chart.
- Remove and add back the first one.
- Move the mouse over the chart.

Workaround:
class MyChartTrackballController : ChartTrackballController
{
    protected override string GetPointText(DataPoint point)
    {
        if (point.Presenter != null)
        {
            return base.GetPointText(point);
        }
        else
        {
            CategoricalDataPoint p = point as CategoricalDataPoint;
            if (p!= null)
            {
                return p.Value.ToString();
            }
        }
        return "";
    }
}