a code sample that cause the nullpointerexception would be helpful here.

I figured out that even the smallest code causes a NullPointerException:

1. import controlP5.*;
   
   ControlP5 controlP5;
   
   void setup() {
     size(800, 800);
     ControlP5 controlP5 = new ControlP5(this);
     controlP5.setAutoDraw(false);
     controlP5.addTextlabel("label", "A SINGLE TESTLABEL.", 20, 134);
   }
   
   void draw()
   {
     background(0);
     controlP5.draw();
   }
   
   

you should not declare controlP5 inside setup again since you defined it already outside as a global variable, which is the right thing to do in your case. 'Local over global', your local controlP5 variable inside setup will override your global controlP5 variable and therefore will only exist inside setup but will be null for draw (since your global variable will not be initialized).

Thanks, that was such a obvious mistake.
I checked everything, except of this.