Uncaught TypeError: Невозможно установить свойство 'onmousedown' для null

Question

Я пытаюсь заставить мои кнопки работать, но они не вызывают "onmouse" и возвращают null

кнопки такие:

               <button type="button" id='right' class='right' name="button">

               <button type="button" id='left' class='left' name="button">

Javascript:

var curpage = 1;
var sliding = false;
var click = true;
var left = document.getElementById("left");
var right = document.getElementById("right");
var pagePrefix = "slide";
var pageShift = 500;
var transitionPrefix = "circle";
var svg = true;

document.onkeydown = checkKey;




function leftSlide() {
     if (click) {
          if (curpage == 1) curpage = 5;
          console.log("woek");
          sliding = true;
          curpage--;
          svg = true;
          click = false;
          for (k = 1; k <= 4; k++) {
               var a1 = document.getElementById(pagePrefix + k);
               a1.className += " tran";
          }
          setTimeout(() => {
               move();
          }, 200);
          setTimeout(() => {
               for (k = 1; k <= 4; k++) {
                    var a1 = document.getElementById(pagePrefix + k);
                    a1.classList.remove("tran");
               }
          }, 1400);
     }
}

function rightSlide() {
     if (click) {
          if (curpage == 4) curpage = 0;
          console.log("woek");
          sliding = true;
          curpage++;
          svg = false;
          click = false;
          for (k = 1; k <= 4; k++) {
               var a1 = document.getElementById(pagePrefix + k);
               a1.className += " tran";
          }
          setTimeout(() => {
               move();
          }, 200);
          setTimeout(() => {
               for (k = 1; k <= 4; k++) {
                    var a1 = document.getElementById(pagePrefix + k);
                    a1.classList.remove("tran");
               }
          }, 1400);
     }
}

function move() {
     if (sliding) {
          sliding = false;
          if (svg) {
               for (j = 1; j <= 9; j++) {
                    var c = document.getElementById(transitionPrefix + j);
                    c.classList.remove("steap");
                    c.setAttribute("class", transitionPrefix + j + " streak");
                    console.log("streak");
               }
          } else {
               for (j = 10; j <= 18; j++) {
                    var c = document.getElementById(transitionPrefix + j);
                    c.classList.remove("steap");
                    c.setAttribute("class", transitionPrefix + j + " streak");
                    console.log("streak");
               }
          }

          // for(k=1;k<=4;k++){
          //   var a1 = document.getElementById(pagePrefix + k);
          //   a1.className += ' tran';
          // }

          setTimeout(() => {
               for (i = 1; i <= 4; i++) {
                    if (i == curpage) {
                         var a = document.getElementById(pagePrefix + i);
                         a.className += " up1";
                    } else {
                         var b = document.getElementById(pagePrefix + i);
                         b.classList.remove("up1");
                    }
               }
               sliding = true;
          }, 600);
          setTimeout(() => {
               click = true;
          }, 1700);

          setTimeout(() => {
               if (svg) {
                    for (j = 1; j <= 9; j++) {
                         var c = document.getElementById(transitionPrefix + j);
                         c.classList.remove("streak");
                         c.setAttribute("class", transitionPrefix + j + " steap");
                    }
               } else {
                    for (j = 10; j <= 18; j++) {
                         var c = document.getElementById(transitionPrefix + j);
                         c.classList.remove("streak");
                         c.setAttribute("class", transitionPrefix + j + " steap");
                    }
                    sliding = true;
               }
          }, 850);
          setTimeout(() => {
               click = true;
          }, 1700);
     }
}

// null here
 left.onmousedown = () => {
     leftSlide();
 };

 right.onmousedown = () => {
      rightSlide();
 };
function checkKey(e) {

     e = e || window.event;

     if (e.keyCode == 37) {
          leftSlide();
     } else if (e.keyCode == 39) {
          rightSlide();
     }
};

Html:

  <!-- Slide comienzo -->

      <body>
         <div class='parent'>
            <div class='slider'>
               <button type="button" id='right' class='right' name="button">

            <svg version="1.1" id="Capa_1" width='40px' height='40px ' xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
              viewBox="0 0 477.175 477.175" style="enable-background:new 0 0 477.175 477.175;" xml:space="preserve">
            <g>
             <path style='fill: #9d9d9d;' d="M360.731,229.075l-225.1-225.1c-5.3-5.3-13.8-5.3-19.1,0s-5.3,13.8,0,19.1l215.5,215.5l-215.5,215.5
               c-5.3,5.3-5.3,13.8,0,19.1c2.6,2.6,6.1,4,9.5,4c3.4,0,6.9-1.3,9.5-4l225.1-225.1C365.931,242.875,365.931,234.275,360.731,229.075z
               "/>
            </g>

            </svg>

            </button>
               <button type="button" id='left' class='left' name="button">
            <svg version="1.1" id="Capa_2" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
              viewBox="0 0 477.175 477.175" style="enable-background:new 0 0 477.175 477.175;" xml:space="preserve">
            <g>
             <path style='fill: #9d9d9d;' d="M145.188,238.575l215.5-215.5c5.3-5.3,5.3-13.8,0-19.1s-13.8-5.3-19.1,0l-225.1,225.1c-5.3,5.3-5.3,13.8,0,19.1l225.1,225
               c2.6,2.6,6.1,4,9.5,4s6.9-1.3,9.5-4c5.3-5.3,5.3-13.8,0-19.1L145.188,238.575z"/>
            </g>
            </svg>
            </button>
               <svg id='svg2' class='up2' xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
           <circle id='circle1' class='circle1 steap' cx="34px" cy="49%" r="20"  />
           <circle id='circle2' class='circle2 steap' cx="34px" cy="49%" r="100"  />
           <circle id='circle3' class='circle3 steap' cx="34px" cy="49%" r="180"  />
           <circle id='circle4' class='circle4 steap' cx="34px" cy="49%" r="260"  />
           <circle id='circle5' class='circle5 steap' cx="34px" cy="49%" r="340"  />
           <circle id='circle6' class='circle6 steap' cx="34px" cy="49%" r="420"  />
           <circle id='circle7' class='circle7 steap' cx="34px" cy="49%" r="500"  />
           <circle id='circle8' class='circle8 steap' cx="34px" cy="49%" r="580"  />
           <circle id='circle9' class='circle9 steap' cx="34px" cy="49%" r="660"  />
         </svg>
               <svg id='svg1' class='up2' xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
           <circle id='circle10' class='circle10 steap' cx="648px" cy="49%" r="20"  />
           <circle id='circle11' class='circle11 steap' cx="648px" cy="49%" r="100"  />
           <circle id='circle12' class='circle12 steap' cx="648px" cy="49%" r="180"  />
           <circle id='circle13' class='circle13 steap' cx="648px" cy="49%" r="260"  />
           <circle id='circle14' class='circle14 steap' cx="648px" cy="49%" r="340"  />
           <circle id='circle15' class='circle15 steap' cx="648px" cy="49%" r="420"  />
           <circle id='circle16' class='circle16 steap' cx="648px" cy="49%" r="500"  />
           <circle id='circle17' class='circle17 steap' cx="648px" cy="49%" r="580"  />
           <circle id='circle18' class='circle18 steap' cx="648px" cy="49%" r="660"  />
         </svg>
               <div id='slide1' class='slide1 up1'>Drake</div>
               <div id='slide2' class='slide2'>BEACH</div>
               <div id='slide3' class='slide3'>FOREST</div>
               <div id='slide4' class='slide4'>DESERT</div>
            </div>
         </div>

      </body>

      <!-- slide fin -->

Я надеюсь, что мои кнопки вызывают каждую из них со своим идентификатором для соответствующей функции, но кажется, что по какой-то причине это не происходит.браузер выдает мне эту ошибку:

Uncaught TypeError: Невозможно установить свойство 'onmousedown' для null в файле: /// C: / Users / Diego / Desktop / New% 20Portfolio% 20 (1) / New%20Портфолио / src / js / script.js: 128: 19 (анонимно) @ script.js: 128