Just to contribute, this is the first time I've also experienced now an issue with BestFit. Here is my stack trace.
Object reference not set to an instance of an object.
at Telerik.WinControls.UI.StretchColumnLayoutHelper.StartColumnResize(GridViewColumn column)
at Telerik.WinControls.UI.TableViewRowLayout.StartColumnResize(GridViewColumn column)
at Telerik.WinControls.UI.TableViewRowLayout.StretchColumn(GridViewColumn column, Int32 desiredWidth)
at Telerik.WinControls.UI.BestFitHelper.SetColumnWidth(GridViewColumn column, Single desiredWidth)
at Telerik.WinControls.UI.BestFitHelper.BestFitColumnsCore()
at Telerik.WinControls.UI.BestFitHelper.ProcessRequests()
at Telerik.WinControls.UI.RowsContainerElement.MeasureOverride(SizeF availableSize)
at Telerik.WinControls.RadElement.MeasureCore(SizeF availableSize)
at Telerik.WinControls.RadElement.Measure(SizeF availableSize)
at Telerik.WinControls.UI.ScrollViewElement`1.MeasureView(SizeF availableSize)
at Telerik.WinControls.UI.ScrollViewElement`1.MeasureOverride(SizeF availableSize)
at Telerik.WinControls.UI.GridTableElement.MeasureOverride(SizeF availableSize)
at Telerik.WinControls.RadElement.MeasureCore(SizeF availableSize)
at Telerik.WinControls.RadElement.Measure(SizeF availableSize)
at Telerik.WinControls.Layouts.DockLayoutPanel.MeasureOverride(SizeF constraint)
at Telerik.WinControls.RadElement.MeasureCore(SizeF availableSize)
at Telerik.WinControls.RadElement.Measure(SizeF availableSize)
at Telerik.WinControls.UI.LightVisualElement.MeasureElements(SizeF availableSize, SizeF clientSize, Padding borderThickness)
at Telerik.WinControls.UI.LightVisualElement.MeasureOverride(SizeF availableSize)
at Telerik.WinControls.RadElement.MeasureCore(SizeF availableSize)
at Telerik.WinControls.RadElement.Measure(SizeF availableSize)
at Telerik.WinControls.RootRadElement.MeasureOverride(SizeF availableSize)
at Telerik.WinControls.RootRadElement.MeasureCore(SizeF availableSize)
at Telerik.WinControls.RadElement.Measure(SizeF availableSize)
at Telerik.WinControls.ComponentLayoutElementTree.PerformInnerLayout(Boolean performMeasure, Int32 x, Int32 y, Int32 width, Int32 height)
at Telerik.WinControls.RadControl.GetInitialDesiredSize(Size availableSize, Boolean useNewLayoutSystem)
at Telerik.WinControls.RadControl.OnLoad(Size desiredSize)
at Telerik.WinControls.RadControl.LoadElementTree(Size desiredSize)
at Telerik.WinControls.RadControl.OnCreateControl()
at System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
at System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
at System.Windows.Forms.Control.CreateControl()
at System.Windows.Forms.Control.SetVisibleCore(Boolean value)
at System.Windows.Forms.Control.set_Visible(Boolean value)
at Telerik.WinControls.UI.RadPageViewPage.set_Visible(Boolean value)
at Telerik.WinControls.UI.RadPageViewElement.SetSelectedContent(RadPageViewItem item)
at Telerik.WinControls.UI.RadPageViewElement.SetSelectedItem(RadPageViewItem item)
at Telerik.WinControls.UI.RadPageViewElement.OnSelectedPageChanged(RadPageViewEventArgs e)
at Telerik.WinControls.UI.RadPageView.SetSelectedPage(RadPageViewEventArgs e)
at Telerik.WinControls.UI.RadPageView.set_SelectedPage(RadPageViewPage value)
at Telerik.WinControls.UI.RadPageViewElement.SelectItem(RadPageViewItem item)
at Telerik.WinControls.UI.RadPageViewElement.OnItemMouseDown(RadPageViewItem sender, MouseEventArgs e)
at Telerik.WinControls.UI.RadPageViewStripElement.OnItemMouseDown(RadPageViewItem sender, MouseEventArgs e)
at Telerik.WinControls.UI.RadPageViewItem.OnMouseDown(MouseEventArgs e)
at Telerik.WinControls.RadElement.OnCLREventsRise(RoutedEventArgs args)
at Telerik.WinControls.RadElement.OnBubbleEvent(RadElement sender, RoutedEventArgs args)
at Telerik.WinControls.RadItem.OnBubbleEvent(RadElement sender, RoutedEventArgs args)
at Telerik.WinControls.RadElement.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
at Telerik.WinControls.RadItem.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
at Telerik.WinControls.RadElement.RaiseRoutedEvent(RadElement sender, RoutedEventArgs args)
at Telerik.WinControls.RadElement.DoMouseDown(MouseEventArgs e)
at Telerik.WinControls.ComponentInputBehavior.OnMouseDown(MouseEventArgs e)
at Telerik.WinControls.RadControl.OnMouseDown(MouseEventArgs e)
at System.Windows.Forms.Control.WmMouseDown(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
at Telerik.WinControls.RadControl.WndProc(Message& m)
at Telerik.WinControls.RadNCEnabledControl.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
as you've said, this only happens in Fill mode. I'll also look into this as we've never had this issue before.