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

i need a help ! how to solve in this error

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at RECOVER_assignment1_crossword_fla::MainTimeline/Play()[RECOVER_assignment1_crossword_fla.MainTimeline::frame3:11] 

here is my code: 

import flash.events.Event;

stop();
var score: int = 0;
var correctInc: int = 10;
score_txt_.text=score.toString();

stage.addEventListener(Event.ENTER_FRAME, Play);
function Play(e:Event) :void {

if (emotional_e_txt.text == ("E").toString() &&
emotional_m_txt.text == ("M").toString() &&
emotional_o_txt.text == ("O").toString() &&
emotional_t_txt.text == ("T").toString() &&
emotional_i_txt.text == ("I").toString() &&
emotional_o1_txt.text == ("O").toString() &&
emotional_n_txt.text == ("N").toString() &&
emotional_a_txt.text == ("A").toString() &&
emotional_l_txt.text == ("L").toString()) {
emobox_mc.gotoAndStop(3);
score = correctInc;

} else if(emotional_e_txt.text == ("").toString() &&
emotional_m_txt.text == ("").toString() &&
emotional_o_txt.text == ("").toString() &&
emotional_t_txt.text == ("").toString() &&
emotional_i_txt.text == ("").toString() &&
emotional_o1_txt.text == ("").toString() &&
emotional_n_txt.text == ("").toString() &&
emotional_a_txt.text == ("").toString() &&
emotional_l_txt.text == ("").toString()){
emobox_mc.gotoAndStop(1);
} else {
emobox_mc.gotoAndStop(2);
}


if (depresion_d_txt.text == ("D").toString() &&
depresion_p_txt.text == ("P").toString() &&
depresion_r_txt.text == ("R").toString() &&
depresion_e_txt.text == ("E").toString() &&
depresion_s_txt.text == ("S").toString() &&
depresion_i_txt.text == ("I").toString() &&
depresion_o_txt.text == ("O").toString() &&
depresion_n_txt.text == ("N").toString()) {
depbox_mc.gotoAndStop(3);
score += correctInc;

} else if (depresion_d_txt.text == ("").toString() &&
depresion_p_txt.text == ("").toString() &&
depresion_r_txt.text == ("").toString() &&
depresion_e_txt.text == ("").toString() &&
depresion_s_txt.text == ("").toString() &&
depresion_i_txt.text == ("").toString() &&
depresion_o_txt.text == ("").toString() &&
depresion_n_txt.text == ("").toString()) {
depbox_mc.gotoAndStop(1);
} else {
depbox_mc.gotoAndStop(2);
}

if (fatigue_f_txt.text == ("F").toString() &&
fatigue_i_txt.text == ("I").toString() &&
fatigue_t_txt.text == ("T").toString() &&
fatigue_g_txt.text == ("G").toString() &&
fatigue_u_txt.text == ("U").toString() &&
fatigue_e_txt.text == ("E").toString()) {
fatbox_mc.gotoAndStop(3);
score += correctInc;

} else if (fatigue_f_txt.text == ("").toString() &&
fatigue_t_txt.text == ("").toString() &&
fatigue_i_txt.text == ("").toString() &&
fatigue_g_txt.text == ("").toString() &&
fatigue_u_txt.text == ("").toString() &&
fatigue_e_txt.text == ("").toString()) {
fatbox_mc.gotoAndStop(1);
} else {
fatbox_mc.gotoAndStop(2);
}

if (medication_m_txt.text == ("M").toString() &&
medication_e_txt.text == ("E").toString() &&
medication_d_txt.text == ("D").toString() &&
medication_c_txt.text == ("C").toString() &&
medication_a_txt.text == ("A").toString() &&
medication_t_txt.text == ("T").toString() &&
medication_i_txt.text == ("I").toString() &&
medication_o_txt.text == ("O").toString() &&
medication_n_txt.text == ("N").toString()) {
medbox_mc.gotoAndStop(3);
score += correctInc;

} else if (medication_m_txt.text == ("").toString() &&
medication_e_txt.text == ("").toString() &&
medication_d_txt.text == ("").toString() &&
medication_c_txt.text == ("").toString() &&
medication_a_txt.text == ("").toString() &&
medication_t_txt.text == ("").toString() &&
medication_i_txt.text == ("").toString() &&
medication_o_txt.text == ("").toString() &&
medication_n_txt.text == ("").toString()) {
medbox_mc.gotoAndStop(1);
} else {
medbox_mc.gotoAndStop(2);
}

if (physical_p_txt.text == ("P").toString() &&
physical_h_txt.text == ("H").toString() &&
physical_y_txt.text == ("Y").toString() &&
physical_s_txt.text == ("S").toString() &&
physical_c_txt.text == ("C").toString() &&
physical_a_txt.text == ("A").toString() &&
physical_l_txt.text == ("L").toString()) {
phybox_mc.gotoAndStop(3);
score += correctInc;

} else if (physical_p_txt.text == ("").toString() &&
physical_h_txt.text == ("").toString() &&
physical_y_txt.text == ("").toString() &&
physical_s_txt.text == ("").toString() &&
physical_c_txt.text == ("").toString() &&
physical_a_txt.text == ("").toString() &&
physical_l_txt.text == ("").toString()) {
phybox_mc.gotoAndStop(1);
} else {
phybox_mc.gotoAndStop(2);
}

score_txt_.text=score.toString();
}

done_btn.addEventListener(MouseEvent.CLICK, goNext);

function goNext(event:MouseEvent):void
{
gotoAndStop(4);
}

i hope someone can help me. Thank you