Add el check for epic swimlanes

ee/app/assets/javascripts/boards/toggle_epics_swimlanes.js

@@ -2,9 +2,15 @@ import Vue from 'vue';
 import ToggleEpicsSwimlanes from './components/toggle_epics_swimlanes.vue';
 import store from '~/boards/stores';
 
-export default () =>
-  new Vue({
-    el: document.getElementById('js-board-epics-swimlanes-toggle'),
+export default () => {
+  const el = document.getElementById('js-board-epics-swimlanes-toggle');
+
+  if (!el) {
+    return null;
+  }
+
+  return new Vue({
+    el,
     components: {
       ToggleEpicsSwimlanes,
     },
@@ -13,3 +19,4 @@ export default () =>
       return createElement(ToggleEpicsSwimlanes);
     },
   });
+}