新手问题：NetBeans出现java.lang.instantiationexception异常

源代码是：
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.TextBox;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.Display;
public abstract class Midlet extends MIDlet implements CommandListener {
private Form fm;
private TextBox tb;
private Command ok_cmd;
private Command cancel_cmd;

public Midlet()
{
fm=new Form("form1");
tb=new TextBox("textbox",null,20,TextField.ANY);
ok_cmd=new Command("OK",Command.OK,0);
cancel_cmd=new Command("Cancel",Command.CANCEL,0);
}
public void startApp() {
Display.getDisplay(this).setCurrent(fm);
fm.append("Hello\n");
fm.append("This is a form");
fm.addCommand(ok_cmd);
fm.addCommand(cancel_cmd);
fm.setCommandListener(this);
}

public void pauseApp() {
}

public void destroyApp(boolean unconditional) {
}
public void commandaction(Command command ,Displayable disp){
if(command==ok_cmd)
{
Display.getDisplay(this).setCurrent(tb);
}
}
}