_mainFrame is NULL in NavigationService

[floppydisk525]

The frame has a navigationService Property associated with it and I tried to copy Laurent's navigation class example from a WindowsPhone example and one line of code doesn't seem to be working like it should, at least I can't get it to work. From Laurent's NavigationService example here. The following line needs to be changed:

_mainFrame = Application.Current.RootVisual as PhoneApplicationFrame;  

I tried changing it to with help from stackoverflow but _mainFrame is alwyas NULL:

_mainFrame = LogicalTreeHelper.FindLogicalNode(Application.Current.MainWindow, "MainFrameDS") as Frame;

But didn't work, nor did this:

 _mainFrame = Application.Current.MainWindow.FindName("MainFrameDS") as Frame;