Index out of bounds when performing certain changes in the settings dialog

[LogicAndTrick]

http://bugs.sledge-editor.com/Bug/View/7

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index (System.ArgumentOutOfRangeException)
   at System.ThrowHelper.ThrowArgumentOutOfRangeException()
   at Sledge.Editor.Settings.SettingsForm.SelectedGameEngineChanged(Object sender, EventArgs e)
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at System.Windows.Forms.CheckBox.OnCheckedChanged(EventArgs e)
   at System.Windows.Forms.CheckBox.set_CheckState(CheckState value)
   at System.Windows.Forms.CheckBox.OnClick(EventArgs e)
   at System.Windows.Forms.CheckBox.OnMouseUp(MouseEventArgs mevent)