I am use a button and when u click this button a text field is automatic created and button status is automatic disable but if u click on disable button text field is automatic delete and button status is changed to enable.i going through this process.

public class ConfigurationScreen extends MainScreen implements FieldChangeListener{

        TextField tf_text;
        tf_text =  new   

   TextField(TextField.TYPE_PLAIN,img_text[1],img_text[0],TextField.FIELD_HCENTER);

   tf_text.setWidth(Display.getWidth()/2+20);

   ImageButton btn_en;

 btn_en = new ImageButton(imgs_tmintrvl1,"enable",ImageButton.FIELD_HCENTER);

  ImageButton btn_dis;

 btn_dis=new ImageButton(imgs_tmintrvl1,"Disable",ImageButton.FIELD_HCENTER); 

add(btn_en);

btn_en.setChangeListener(this);

public void fieldChanged(Field field, int context) {

         if( field==btn_en)
            {


                    delete(btn_en);
                    insert(btn_dis, 4);
                    insert(tf_text, 5);


                System.out.println(ex);

            }else if (field == btn_dis){


                delete(btn_dis);
                delete(tf_text);
                insert(btn_en, 4);


                System.out.println("Disable Button="+ex);


            }
    }


But when i run this code i am getting null pointerr exception please help me where i   am making mistake.
Thanks in Advance