White Window when using Window.Show()

Hello,

I have made a window Window1 with a TextBlock "Loading..." so i can display it when the code is slow to run with 

Window1 window1 = new Window1();
window1.Show();

And close it after the code has run.

The problem is that it displays a window completely white. Whereas it works with "window1.ShowDialog();" 

But i can't use ShowDialog. (I need the code to keep running after showing the window)

I don't understand why ShowDialog() works and Show(); doesnt 

Thx for helping