Added guards for missing codemirror elements when destroying markdown cards

no issue

- if a markdown card is created and destroyed before the simplemde+codemirror instance is fully initialised we could throw errors due to simplemde calling methods on objects that don't yet exist during teardown
- we also had issues with the `_applyToolbarStyles()` method being called async by event handlers after the component was destroyed

ghost/admin/app/components/gh-simplemde.js

@@ -72,7 +72,9 @@ export default class GhSimplemde extends TextArea {
     }
 
     willDestroyElement() {
-        this._editor.toTextArea();
+        if (this._editor?.codemirror) {
+            this._editor.toTextArea();
+        }
         delete this._editor;
         super.willDestroyElement(...arguments);
     }

ghost/admin/lib/koenig-editor/addon/components/koenig-card-markdown.js

@@ -154,7 +154,11 @@ export default class KoenigCardMarkdown extends Component {
     }
 
     _applyToolbarStyles() {
-        let toolbar = this.element.querySelector('.editor-toolbar');
+        if (this.isDestroyed || this.isDestroying) {
+            return;
+        }
+
+        let toolbar = this.element?.querySelector('.editor-toolbar');
 
         if (!toolbar) {
             return;