Why DispatcherTimer does not click?

Question

This WPF application (VisualStudio 2015 Community and .NER Framework 4.5.1) is

expected, when a button is pressed, to write on the console, at a rate of one per second,

all the items contained in List<string> playList.

In fact, the DispatcherTimer_Tick method is never called and consequently playListIndex is never  called and the program is stucked in the while loop.

Two files are attached:

      MainWindow.xaml.cs

      MainWindow.xaml 

 

Thank you in advance for any help