System.NullReferenceException Windows Store apps C#

public class MainViewModel : ViewModelBase { private MediaClass mediaTest; public MediaClass MediaTest { get { return mediaTest; } set {Set("MediaTest",ref mediaTest,value); } } public MainViewModel() {             mediaTest = new MediaClass(); MediaPlayCommand = new RelayCommand(Play); } public RelayCommand MediaPlayCommand { get; private set; } public async void Play() { FileOpenPicker filePicker = new FileOpenPicker();             filePicker.SuggestedStartLocation = PickerLocationId.VideosLibrary;              filePicker.FileTypeFilter.Add(".wmv");             filePicker.FileTypeFilter.Add(".mp4");             filePicker.ViewMode = PickerViewMode.Thumbnail; StorageFile localVideo = await filePicker.PickSingleFileAsync(); if (localVideo != null) { // Open the selected file and set it as the MediaElement's source IRandomAccessStream stream = await localVideo.OpenAsync(FileAccessMode.Read); this.MediaTest.MediaPaly = MediaTest.MediaPaly; MediaTest.MediaPaly.SetSource(stream, localVideo.ContentType); } } }

 x:Class="MediaTest.MainPage"     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"     xmlns:local="using:MediaTest"     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"     mc:Ignorable="d" DataContext="{Binding Main, Mode=OneWay, Source={StaticResource Locator}}">    

MediaTest.MediaPaly.SetSource(stream, localVideo.ContentType); // System.NullReferenceException