Fix for bug introduced in commit

IDE is throwing following exception...

System.NullReferenceException: Object reference not set to an instance of an object
at MonoDevelop.Components.Docking.DockBarItem.<Present>m__2 ()
[0x00027] in main/src/core/MonoDevelop.Ide/MonoDevelop.Components.Docking/DockBarItem.cs:278
at GLib.Timeout+TimeoutProxy.Handler () [0x00000] in <filename
unknown>:0

Added null ref check on autoShowFrame.ContainerWindow.