Tengo un problema importante durante la carga de la extensión en el programa. Obtengo una excepción ya que no hay un constructor válido.

El problema está en la línea:

 ekstensja =  (ArrayList<Dydaktyk>) ois.readObject();

Me sale algo así:

java.io.InvalidClassException: Dydaktyk; no valid constructor
    at java.io.ObjectStreamClass$ExceptionInfo.newInvalidClassException(Unknown Source)
    at java.io.ObjectStreamClass.checkDeserialize(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at java.util.ArrayList.readObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at Dydaktyk.wczytajEkstensje(Dydaktyk.java:81)
    at Dydaktyk.<clinit>(Dydaktyk.java:69)
    at java.io.ObjectStreamClass.hasStaticInitializer(Native Method)
    at java.io.ObjectStreamClass.computeDefaultSUID(Unknown Source)
    at java.io.ObjectStreamClass.access$100(Unknown Source)
    at java.io.ObjectStreamClass$1.run(Unknown Source)
    at java.io.ObjectStreamClass$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.io.ObjectStreamClass.getSerialVersionUID(Unknown Source)
    at java.io.ObjectStreamClass.initNonProxy(Unknown Source)
    at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
    at java.io.ObjectInputStream.readClassDesc(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.defaultReadFields(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at java.util.ArrayList.readObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.defaultReadFields(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at java.util.ArrayList.readObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at Przedmiot.wczytajEkstensje(Przedmiot.java:99)
    at Przedmiot.<clinit>(Przedmiot.java:87)
    at GUI.main(GUI.java:100)

static {
    wczytajEkstensje(); // load Extension
}

public static void wczytajEkstensje() {// load extension
    FileInputStream fis = null;
    ObjectInputStream ois = null;
    try {
        fis = new FileInputStream("dydaktyk.ser");
        ois = new ObjectInputStream(fis);
        // here is the problem
        ekstensja =  (ArrayList<Dydaktyk>) ois.readObject();
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } finally {
        try {
            if (ois != null) {
                ois.close();
            }
        } catch (IOException e) {
        }
        try {
            if (fis != null) {
                fis.close();
            }
        } catch (IOException e) {
        }
    }
}

public static void zapiszEkstensje() {// save extension
    FileOutputStream fos = null;
    ObjectOutputStream oos = null;
    try {
        fos = new FileOutputStream("dydaktyk.ser");
        oos = new ObjectOutputStream(fos);

        oos.writeObject(ekstensja); // serialization
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if (oos != null) {
                oos.close();
            }
        } catch (IOException e) {
        }
        try {
            if (fos != null) {
                fos.close();
            }
        } catch (IOException e) {
        }
    }
}