MC-4217: Performance of show/hide Slider on stage is very slow

- Ensure element is present before trying to init WYSIWYG
- Ensure selection is present before attempting to restore

Author: davemacaulay

app/code/Magento/PageBuilder/view/adminhtml/web/js/content-type/slide/preview.js

@@ -159,7 +159,7 @@ export default class Preview extends BasePreview {
             element.focus();
         };
 
-        if (!this.wysiwyg) {
+        if (this.element && !this.wysiwyg) {
             const selection = this.saveSelection();
             this.element.removeAttribute("contenteditable");
             _.defer(() => {
@@ -375,7 +375,7 @@ export default class Preview extends BasePreview {
      * @param {Selection} selection
      */
     private restoreSelection(element: HTMLElement, selection: Selection) {
-        if (window.getSelection) {
+        if (selection && window.getSelection) {
             // Find the original container that had the selection
             const startContainerParent = $(element).find("[data-startContainer]");
             startContainerParent.removeAttr("data-startContainer");