Repaint Window From Another Class?

I am trying to repaint a window from another class. the class Window handles an interface that displays pictures of movies. Via a JMenuBar you can add a new movie and its picture but in order for it to show in the Window i need to repaint certain aspects in the Window class GUI;

The window class constructor:

public Window() {
addComponents();
configurFrame();
addMenu();
}

The functions i want to repaint:

public void repaintWindow() {
this.getContentPane().validate();
this.getContentPane().repaint();
}

From my other class where i add a movie to an ArrayList i have made a "private Window myWindow;" there i call the function repaintWindow via mywindow.repaintWindow(); But this gives me an NullPointerException related to the repaintWindow function.