FIX: display a correct error when attempting to agree on a deferred flag

Previously we would raise a 500 error if a moderator tried to agree on a
flag another moderator deferred.

This can happen cause the UX for flags does not live refresh as flags
are handled

app/controllers/admin/flags_controller.rb

@@ -73,6 +73,14 @@ class Admin::FlagsController < Admin::AdminController
 
     post_action_type = PostAction.post_action_type_for_post(post.id)
 
+    if !post_action_type
+      render_json_error(
+        I18n.t("flags.errors.already_handled"),
+        status: 409
+      )
+      return
+    end
+
     keep_post = ['silenced', 'suspended', 'keep'].include?(params[:action_on_post])
     delete_post = params[:action_on_post] == "delete"
     restore_post = params[:action_on_post] == "restore"

app/models/post_action.rb

@@ -633,7 +633,7 @@ class PostAction < ActiveRecord::Base
 
   def self.post_action_type_for_post(post_id)
     post_action = PostAction.find_by(deferred_at: nil, post_id: post_id, post_action_type_id: PostActionType.notify_flag_types.values, deleted_at: nil)
-    PostActionType.types[post_action.post_action_type_id]
+    PostActionType.types[post_action.post_action_type_id] if post_action
   end
 
   def self.target_moderators

config/locales/server.en.yml

@@ -864,6 +864,9 @@ en:
       read: "Read all"
       write: "Write all"
 
+  flags:
+    errors:
+      already_handled: "Flag was already handled"
   reports:
     default:
       labels:

spec/requests/admin/flags_controller_spec.rb

@@ -34,6 +34,21 @@ RSpec.describe Admin::FlagsController do
   end
 
   context '#agree' do
+    it 'should raise a reasonable error if a flag was deferred and then someone else agreed' do
+      SiteSetting.queue_jobs = false
+
+      _post_action = PostAction.act(user, post_1, PostActionType.types[:spam], message: 'bad')
+
+      post "/admin/flags/defer/#{post_1.id}.json"
+      expect(response.status).to eq(200)
+
+      post "/admin/flags/agree/#{post_1.id}.json", params: { action_on_post: 'keep' }
+      # 409 means conflict which is what is happening here
+      expect(response.status).to eq(409)
+      error = JSON.parse(response.body)["errors"].first
+      expect(error).to eq(I18n.t("flags.errors.already_handled"))
+    end
+
     it 'should be able to agree and keep content' do
       SiteSetting.queue_jobs = false