WpfAppBar.UndockSample does not properly switch

[jschroedl]

Build and run the Itp.WpfAppBar.UndockSample app. Click "Switch DockMode". The app docks, as expected. Click "Switch DockMode". The app un-docks, as expected. Click "Switch DockMode". A copy of the main window is shown instead of docking the current window.

The bug seems to be in the interaction of the DockMode property getter and ShowDockMode() in DockableWindowControl.cs. The getter checks for AppBarHost == null but when we leave the dock mode, this code runs:

                    if (AppBarHost != null)
                    {
                        AppBarHost.Content = null;
                        AppBarHost.Close();
                    }

This does not set AppBarHost to null so after this point, the DockMode will return Docked even though we've closed the appbar.

I think this is the proper fix:

                   if (AppBarHost != null)
                    {
                        AppBarHost.Content = null;
                        AppBarHost.Close();
                        AppBarHost = null;
                    }

A similar change is needed just below that for WindowHost:

                   if (WindowHost != null)
                    {
                        WindowHost.Content = null;
                        WindowHost.Close();
                        WindowHost = null;
                    }

[mgaffigan]

@jschroedl, Thanks for opening the issue! If you would like to submit a PR, I will review and merge.