As hoaz says, you need to initialize your object.
Also, your startThread() method is redundant. Why not just call start?
Anyway, it's considered bad practice to have threads start themselves. So I propose this rewrite of your code:
public class Midlet extends MIDlet {
ThreadClass th = null;
Thread myThread = null;
public void startApp() {
if (th==null) {
th = new ThreadClass();
myThread = new Thread(th);
}
myThread.start();
}
public void pauseApp() {
}
public void destroyApp(boolean unconditional) {
}
}
class ThreadClass implements Runnable{
ThreadClass() {
}
public void run() {
System.out.println("We are in Thread");
}
}