Problem przy zapisie

public List<Wheel> wheels = new ArrayList<Wheel>();
    private List<Rectangle> rectangles = new ArrayList<Rectangle>();
    private List<Polygons> polygons = new ArrayList<Polygons>();

if(source == saveButton)
        {

            try{
                FileOutputStream fs1 = new FileOutputStream("zapis.ser");
                ObjectOutputStream os_weel = new ObjectOutputStream(fs1);

                os_weel.writeObject(wheels);
                os_weel.close();
                fs1.close();

                FileOutputStream fs2 = new FileOutputStream("zapis2.ser");
                ObjectOutputStream os_rec = new ObjectOutputStream(fs2);
                os_rec.writeObject(rectangles);
                os_rec.close();
                fs2.close();

                FileOutputStream fs3 = new FileOutputStream("zapis3.ser");
                ObjectOutputStream os_polly = new ObjectOutputStream(fs3);
                os_polly.writeObject(polygons);
                os_polly.close();
                fs3.close();

            }
            catch (Exception ex){System.out.println("Nie udało się zapisać");}
   }
 if(source == loadButton)
        {
            try {
                ObjectInputStream c1 = new ObjectInputStream(new FileInputStream("zapis.ser"));
                     wheels = (ArrayList<Wheel>)c1.readObject(); //i na tej linijce się sypie
            }
            catch (Exception ex){System.out.println("...");}
       }

Wheel obj = null;
while ((obj = (Wheel) c1.readObject()) != null) {
     wheels.add(obj);
}

 catch (Exception ex){System.out.println("...");}

ex.printStackTrace();