Remove a hitTestObject in a Game

Basically every functions without hitTests are removed without errors, but all the hittestobjects that i have in the end game frame says that it cannot acess a null hitTestObject. One of the hitTests i have is this one:

if (this.hitTestObject(_root.anulpowerups)) {
this.parent.removeChild(this);
removeEventListener(Event.ENTER_FRAME, checkCollision);

            }

How can i remove them successfully?