Throw error rather than return `null`

src/lib/data_structures/publication_tree.ts

@@ -136,10 +136,10 @@ export class PublicationTree implements AsyncIterable<NDKEvent> {
    * @returns Returns an array of events in the addressed event's hierarchy, beginning with the
    * root and ending with the addressed event.
    */
-  async getHierarchy(address: string): Promise<NDKEvent[] | null> {
+  async getHierarchy(address: string): Promise<NDKEvent[]> {
     let node = this.#nodes.get(address);
     if (!node) {
-      return null;
+      throw new Error(`PublicationTree: Node with address ${address} not found.`);
     }
 
     const hierarchy: NDKEvent[] = [this.#events.get(address)!];