fix : Frontend error

frontend/src/pages/ActivityPage.tsx

@@ -422,7 +422,7 @@ export function ActivityPage({ onStartTask }: ActivityPageProps) {
                           {/* Show who is working on this task */}
                           {task.working_by && (
                             <p className="text-blue-300 text-xs mt-1">
-                              {isCurrentUserWorking ? 'You are' : `${getWorkerName(task.working_by)} is`} working
+                              {isCurrentUserWorking ? 'You are' : `${getWorkerName(task.working_by ?? "")} is`} working
                             </p>
                           )}
                           {isOtherUserWorking ? (
@@ -430,7 +430,7 @@ export function ActivityPage({ onStartTask }: ActivityPageProps) {
                               disabled
                               className="mt-2 w-full px-3 py-1.5 bg-gray-500/20 text-gray-400 text-xs font-medium rounded cursor-not-allowed"
                             >
-                              {getWorkerName(task.working_by)} is working
+                              {getWorkerName(task.working_by ?? "")} is working
                             </button>
                           ) : (
                             <button
@@ -556,7 +556,7 @@ export function ActivityPage({ onStartTask }: ActivityPageProps) {
                         </p>
                       </div>
                     ))
-                  )}
+                  }
                 </div>
               </div>
             </div>