FIX: avoid error when attempting to show ai helper to anon (#549)

Also adds an extra guard around ai helper context positioning.

assets/javascripts/discourse/connectors/after-d-editor/ai-helper-context-menu.js

@@ -275,6 +275,11 @@ export default class AiHelperContextMenu extends Component {
   @afterRender
   positionContextMenu() {
     this._contextMenu = document.querySelector(".ai-helper-context-menu");
+
+    if (!this._dEditorInput || !this._contextMenu) {
+      return;
+    }
+
     this.caretCoords = getCaretPosition(this._dEditorInput, {
       pos: caretPosition(this._dEditorInput),
     });

assets/javascripts/discourse/lib/show-ai-helper.js

@@ -15,7 +15,7 @@ export function showComposerAIHelper(outletArgs, helper, featureType) {
 export function showPostAIHelper(outletArgs, helper) {
   return (
     _helperEnabled(helper.siteSettings) &&
-    helper.currentUser.can_use_assistant_in_post
+    helper.currentUser?.can_use_assistant_in_post
   );
 }