fix(ThreadChannel): Handle possibly null parent (v13)

#8467 djs v13.10

src/structures/ThreadChannel.js

@@ -283,10 +283,11 @@ class ThreadChannel extends Channel {
    * <info>This only works when the thread started from a message in the parent channel, otherwise the promise will
    * reject. If you just need the id of that message, use {@link ThreadChannel#id} instead.</info>
    * @param {BaseFetchOptions} [options] Additional options for this fetch
-   * @returns {Promise<Message>}
+   * @returns {Promise<Message|null>}
    */
-  fetchStarterMessage(options) {
-    return this.parent.messages.fetch(this.id, options);
+  // eslint-disable-next-line require-await
+  async fetchStarterMessage(options) {
+    return this.parent?.messages.fetch(this.id, options) ?? null;
   }
 
   /**