Add a gantt populated with some data.

Increase column sizes in the text part on a way that an editor is outside the view

Scroll to the editor and open it for edit

Scroll back on a way that the editor is no longer into view => exception is thrown

Workaround: 
        radGanttView1.GanttViewElement.TextViewElement.ColumnScroller.Scrollbar.ValueChanged+=Scrollbar_ValueChanged;

        void Scrollbar_ValueChanged(object sender, EventArgs e)
        {
            radGanttView1.GanttViewElement.EndEdit();
        }