Fix error handling to display them correctly without crashing

frontend/src/mock/UserStatsMockService.ts

@@ -19,4 +19,8 @@ export default class UserStatsMockService extends UserStatsService {
     async getStats(): Promise<TableEntry[]> {
         return Promise.resolve(this.entries);
     }
+
+    getStatsWithoutPromise(): TableEntry[] {
+        return this.entries;
+    }
 }