not an object error

In this code, a checkmark appears when each button is clicked. When all 3 buttons are clicked I want the continue arrow to appear. It is on stage, but invisible. I get an error "TypeError: undefined is not an object (evaluating 'this.continueArrow.visible = true')".

this.atmosphere_but.addEventListener("click", fl_ClickToGoToAndStopFromFrame208.bind(this));  var checks = 0;  function checkChecks() {      //alert(checks);      if (checks >=3) {            this.continueArrow.visible = true;        } }  function fl_ClickToGoToAndStopFromFrame208() {      this.gotoAndStop(208);      this.check1.visible = true;      checks = checks+1;      checkChecks();       }   this.crust_but.addEventListener("click", fl_ClickToGoToAndStopFromFrame209.bind(this));  function fl_ClickToGoToAndStopFromFrame209() {      this.gotoAndStop(209);      this.check2.visible = true;      checks = checks+1;      checkChecks(); }   this.mantle_but.addEventListener("click", fl_ClickToGoToAndStopFromFrame210.bind(this));  function fl_ClickToGoToAndStopFromFrame210() {      this.gotoAndStop(210);      this.check3.visible = true;      checks = checks+1;      checkChecks(); }