Processing - ArrayList.add nullpointerexception

Question

here's the entire processing file -

float camX = 0;
float camY = 0;
float camZ = 0;
float camS = 1;
float camA = 0;

float dist(float x,float y){
  return sqrt(pow(x,2)+pow(y,2));
}
float angle(float x,float y){
  if(x<0){
        return (3*PI/2)-atan(y/x);
    } else {
        return (PI/2)-atan(y/x);
    }
}



class Vert{
  float x;
  float z;
  Vert(float x, float z){
    this.x=x;
    this.z=z;
  }
  float getX(){
    return x;
  }
  float getZ(){
    return z;
  }
}
class Shape{
  float x;
  float y;
  float z;
  ArrayList<Vert> verts;
  float h;
 Shape(float x,float y,float z,ArrayList<Vert> verts,float h){
   this.x=x;
   this.y=y;
   this.z=z;
   this.verts = verts;
   this.h = h;
 }
 float getX(){
   return x;
 }
 float getY(){
   return y;
 }
  float getZ(){
   return z;
 }
 ArrayList<Vert> getVerts(){
   return verts; 
 }
 float getHeight(){
 return h;
}
ArrayList<Vert> getSlice(float sy){
   if(sy>=y&&sy<=y+height){
      return verts;
    } else {
     return null; 
    }
}

}


ArrayList<Shape> shapes;


Vert transVert(float vx,float vy,float vz,float cx,float cy,float cz,float cs,float ca){
  float a = angle(cx-vx,cz-vz);
  float d = dist(cx-vx,cz-vz);
  println(a+" , "+d);
  return new Vert((width/2)+(cx+sin((a+ca))*d*cs),(height/2)-(cz+cy+vy+cos((a+ca))*d*cs));
}


void drawSlice(float sy,float cx,float cy,float cz,float cs,float ca){
  for(int si = 0;si < shapes.size();si ++){
    Shape shape = shapes.get(si);
    ArrayList<Vert> slice = shape.getSlice(sy);
    beginShape();
    for(int i = 0;i < slice.size();i ++){
      Vert vi = transVert(slice.get(i).getX()+shape.getX(),sy+shape.getY(),slice.get(i).getZ()+shape.getZ(),cx,cy,cz,cs,ca);
      vertex(vi.getX(),vi.getZ());
    }
    endShape(CLOSE);
  }
}

void drawSpace(float y1,float y2,float res,float cx,float cy,float cz,float cs,float ca){
   for(float sy = y1;sy < y2;sy += res){
     drawSlice(sy,cx,cy,cz,cs,ca);
   }
}

void setup(){
  size(400,400);
  ArrayList<Vert> vs = new ArrayList<Vert>();
  vs.add(new Vert(-50,0));
  vs.add(new Vert(0,-50));
  vs.add(new Vert(50,0));
  vs.add(new Vert(0,50));
  shapes.add(new Shape(0,0,0,vs,20));
  shapes.add(new Shape(100,0,50,vs,20));
}
void draw(){
  background(255);
  drawSpace(-10,30,5,camX,camY,camZ,camS,camA);
}

i'm getting nullpoint exceptions at the shapes.add lines, but not the vs.add lines? is the spaes array somehow out of range? i need it to be a public arraylist

i've been having lots of trouble with arraylists that i've never had before, hence why i'm using such a messy method of adding individual elements instead of doing it in one line- so i guess any advice would be useful - thanks