Please refer to the attached project. Workakround: call the BeginEdit method in the RadGridView.Click event.
Reason for decline: Calling the BeginEdit method in the handler of the GotFocus event moves the focus from RadGridView to the active editor. When clicking on any other cell (x, y), this will trigger the closing of the editor from the previous active cell. At this point, the GotFocus and the LostFocus events are fired again because the editor of new clicked cell is activated.