Check for existing Turtle object

Listing 5 tests to determine

• if the variable named turtle
• that was declared in Listing 3
• still contains null.

If so, that means that this is the first time that the button has been clicked

• and the Turtle object has not yet been added to the world.

          if(turtle == null){
            turtle = new Turtle(150,150,world);
            turtle.setHeading(90);
            turtle.setShellColor(Color.RED);
            turtle.setBodyColor(Color.BLUE);