1. int screen_w = 640;
2. int screen_h = 480;
3. ArrayList<PGraphics> arrl = new ArrayList<PGraphics>();
4. void setup(){
5.   size(screen_w,screen_h);
6.   background(255,255,255);
7.   arrl.add(createGraphics(screen_w,screen_h));
8. }
9. void draw(){
10.   image(arrl.get(0),0,0);
11. }

Line 10 results in a NullPointerException.
I've tried replacing all PGraphics with PImage, and createGraphics(...) with createImage(...,RGB), and it gives me no problems.