mentions: fix exception when mentioning nonexistent user.

app/logical/concerns/mentionable.rb

@@ -28,7 +28,7 @@ module Mentionable
     text_was = send(:attribute_before_last_save, message_field)
 
     names = DText.parse_mentions(text) - DText.parse_mentions(text_was)
-    users = names.map { |name| User.find_by_name(name) }.uniq
+    users = names.map { |name| User.find_by_name(name) }.compact.uniq
     users = users.without(CurrentUser.user)
 
     users.each do |user|

test/unit/forum_post_test.rb

@@ -66,6 +66,12 @@ class ForumPostTest < ActiveSupport::TestCase
           @forum_post = as(@user) { create(:forum_post, body: "hi from @#{@user.name}") }
         end
       end
+
+      should "not fail when mentioning a nonexistent user" do
+        assert_no_difference("Dmail.count") do
+          @forum_post = as(@user) { create(:forum_post, body: "hi from @nonamethanks") }
+        end
+      end
     end
 
     context "that belongs to a topic with several pages of posts" do