General Information:
- works until 2012.2.726.00 Q2 2012 SP1 version of our controls
- does not work after 2012.3.1017.00 Q3 2012 version of our controls

Steps to reproduce:
1) Add RadDock control
2) Show Next and Previous Buttons of the default DocumentTabStrip:

        private void Form1_Load(object sender, EventArgs e)
        {
            this.radDock1.DockTabStripNeeded += new DockTabStripNeededEventHandler(radDock1_DockTabStripNeeded);
            
            tabStrip = this.radDock1.GetDefaultDocumentTabStrip(false);
            tabStrip.TabStripElement.StripButtons = StripViewButtons.All;
            DockLayoutPanel dockPanel = tabStrip.RootElement.Children[0].Children[3] as DockLayoutPanel;
            if(dockPanel != null)    
            {
                dockPanel.ZIndex = -1;
            }
            StripViewItemContainer stripContainer = tabStrip.TabStripElement.Children[0] as StripViewItemContainer;
            if (stripContainer != null)
            {
                stripContainer.Padding = new Padding(stripContainer.Padding.Left, stripContainer.Padding.Top, 0, stripContainer.Padding.Bottom);
            }
        }

        void radDock1_DockTabStripNeeded(object sender, DockTabStripNeededEventArgs e)
        {
            if(e.DockType == DockType.Document)
            {
                e.Strip = new CustomDocumentTabStrip();
            }
        }            

Expected Result: Navigate through all document windows by using the Next and Previous Button beside OverflowDropdownButton
Actual Result: NullReferenceException unhandled by RadPageViewStripElement