index def now return every tasks if requested

app/controllers/api/v1/tasks_controller.rb

   before_action :check_login
 
   def index
-    render json: TaskSerializer.new(Activity.find(params[:activity_id]).tasks).serializable_hash.to_json
+    if !params[:activity_id]
+      render json: TaskSerializer.new(Task.all).serializable_hash.to_json
+    else
+      render json: TaskSerializer.new(Activity.find(params[:activity_id]).tasks).serializable_hash.to_json
+    end
   end
 
   def show

config/routes.rb

       resources :activities do
         resources :tasks
       end
+      resources :tasks
     end
   end
 end