Merge pull request #4 from javascript-info/master

Author: KenjiI

2-ui/1-document/11-coordinates/head.html

@@ -1,8 +1,9 @@
 <script>
 document.addEventListener('DOMContentLoaded', function() {
 
-  document.getElementById('coords-show-mark').onclick = function() {
-    let elem = document.getElementById("coords-show-mark");
+  let elem = document.getElementById('coords-show-mark');
+
+  elem && elem.onclick = function() {
 
     function createMessageUnder(elem, text) {
       let coords = elem.getBoundingClientRect();