WPF Application closes while trying to find static resource

I have some Image components on the page and some static images defined in App.xamp. On startup I need to fill Image components with random static images.

int[] squareImageSources = new int[13]; 
Random rnd = new Random();
Image[] squareImages = new Image[13];

public Page1()
{
    squareImages[0] = i0;
    squareImages[1] = i1;
    squareImages[2] = i2;
    squareImages[3] = i3;
    squareImages[4] = i4;
    squareImages[5] = i5;
    squareImages[6] = i6;
    squareImages[7] = i7;
    squareImages[8] = i8;
    squareImages[9] = i9;
    squareImages[10] = i10;
    squareImages[11] = i11;
    squareImages[12] = i12;
    InitializeComponent();
}

private void UserControl_Loaded(object sender, RoutedEventArgs e)
{
    int randomNumber = rnd.Next(28);
    for (int i = 0; i < 13; i++)
    {
        while (squareImageSources.Contains(randomNumber))
            randomNumber = rnd.Next(28);
        squareImageSources[i] = randomNumber;
        squareImages[i].Source = (BitmapImage)System.Windows.Application.Current.FindResource("s" + Convert.ToString(randomNumber + 1)); //application closes here
    }
}

App.xaml:

<BitmapImage x:Key="s1" UriSource="pack://application:,,,/Resources/Photos/1.png"/>
<BitmapImage x:Key="s2" UriSource="pack://application:,,,/Resources/Photos/2.png"/>
.....................................................
<BitmapImage x:Key="s28" UriSource="pack://application:,,,/Resources/Photos/28.jpg"/>

But application just closes, I recieve no exception. What can be wrong with this code?

UPD:

Tried this way:

try
{
    BitmapImage bi = (BitmapImage)System.Windows.Application.Current.TryFindResource("s" + Convert.ToString(randomNumber + 1));
    squareImages[i].Source = bi; //nullReferenceException
}
catch
{

}

catch catches NullReferenceException. How is that possible? I can use this recource in designer and if works fine.