As I wrote in the thread you found, I am puzzled...
I searched on of my sketches where I used createGraphics with JAVA2D. They are actually numerous. For example:
http://bazaar.launchpad.net/~philho/+junk/Processing/annotate/head:/_QuickExperiments/GradientPaint/GradientPaint.pde
They work without problem.
Even more puzzling: if I take the code of JD, changing it to JAVA2D, I consistently get it to crash (Win7, Java 1.6.0_21, Processing 0191). I ran the code above, OK. I switched back to JD's code, it works!
Looks like a JVM problem... This is rather inconvenient.
For the record, the code that allows to reproduce the crash (on a freshly started PDE) is:

1. PGraphics buffer;
2. float val=0;
3. 
   
4. void setup()
5. {
6.   size(800, 600);
7.   background(0);
8.  
9.   buffer = createGraphics(width, height, JAVA2D);
10. }
11. 
    
12. float angle;
13. float cosine;
14. 
    
15. void draw()
16. {
17.   //Fade everything which is drawn
18.   noStroke();
19.   fill(0, 0, 0, 10);
20.   rect(0, 0, width, height);
21.  
22.   //Draw ellipses
23.   val+= 10;
24.   stroke(196, 5, 23);     //Stroke Color of the ellipse and rectangle/red
25.   noFill();               // No fill color for the ellipse
26.   pushMatrix();
27.   translate(mouseX, mouseY);
28.   ellipseMode(CENTER);
29.   rotate(val);
30.   ellipse(0, 0, 100, 200);
31.   popMatrix();
32.   //Draw rectangles in a buffer so they don't get faded
33.   if (second() % 2 == 0)
34.   {
35.     cosine = cos(val);
36.    
37.     buffer.beginDraw();
38.     buffer.stroke(196,5,23);
39.     buffer.noFill();
40.     buffer.translate(mouseX, mouseY);
41.     buffer.rotate(cosine); 
42.     buffer.translate(width/2, height/2);
43.     //Translating the the width and height, distance the rect. spins away from ellipse.
44.     buffer.rectMode(CENTER);
45.     buffer.rect(0, 0, 115, 115);
46.     buffer.endDraw();
47.   }
48.   image(buffer, 0, 0);
49. }

Cheers phi.lho! Hmmm, so have you got any tips for what I could do? I want to still use the JAVA2D renderer ideally. 

Ok, so I've added: 

I recently ran into this problem trying to use OPENGL 3d to render to a PGraphics.
No matter what I tried, I got a nullpointerexception when calling image(), even when the PGraphics was definitely not null.

After playing around with it a bit, I discovered something: even when I was rendering to a PGraphics, it was still drawing to the screen.

Effectively, my code now draws to the screen successfully without actually using Image(). I don't know if this works in 2D, though.

I also noticed that there was no problem when I set P3D as the renderer instead...

OpenGL has nothing to do with the original thread.

It is a display mode, handled by the video card's hardware, not designed to draw in a buffer. It is stated in the reference.
There are workarounds (VBO, I think) but out of the box you cannot do that.