TypeError: Cannot read property 'checked' of null in ionic 3

please check this i got this error.i am receiving on and off from server and change the button by the "on' ,"off" condition but its works but this error i got

setInterval(function(){

                          peopleData.btnState().subscribe(btnstate => {
                                                 ourthis.zone.run(() => {

                                                 var btni =<HTMLInputElement> document.getElementById("myonoffswitch");




                                                    if(btnstate == "off") {
                                                        
                                                             btni.checked=true;
                                                    }
                                                    else if(btnstate== "on") {
                                                        
                                                            btni.checked=false;
                                                    }
            
                    }); 
                         });

<div class="onoffswitch">
                  <input type="checkbox" name="onoffswitch" class="onoffswitch-checkbox" id="myonoffswitch"   checked (click)="onoffButton()">
                  <label class="onoffswitch-label" for="myonoffswitch">
                  <span class="onoffswitch-inner"></span>
                  <span class="onoffswitch-switch"></span>
                  </label>
               </div>

i run this code but i get this error TypeError: Cannot read property 'checked' of null so please help me