Type Error #1009

Hi, I created a movie that has six movie clip symbols on the stage, three of them are animated controlled by ActionScript.  I am trying to created a class file that uses the transition manager to control the remaining movie clip symbols.  When I test the movie I get this error.

TypeError: Error #1009: Cannot access a property or method of a null object reference.

at fl.transitions::TransitionManager$/start()

at math_Exchange()

I can't figure out what's wrong.  Here is the class file.

package  {

import flash.display.MovieClip;
import fl.transitions.*;
import fl.transitions.easing.*;
import flash.events.Event;
import flash.events.MouseEvent;

public class math_Exchange extends MovieClip {
 
 
  public function math_Exchange() {
   // constructor code
   TransitionManager.start(lion1,{type:Fly, direction:Transition.IN, duration:6, easing:Strong.easeOut});
   TransitionManager.start(lion2,{type:Fly, direction:Transition.IN, duration:4, easing:Strong.easeOut});
   TransitionManager.start(lion3,{type:Fly, direction:Transition.IN, duration:2, easing:Strong.easeOut});
  }
}

}

Any help is appreciated