FIX: Safely return from missing post on check_dont_feed_the_trolls (#21238)

lib/composer_messages_finder.rb

@@ -233,7 +233,7 @@ class ComposerMessagesFinder
       if @details[:post_id]
         Post.find_by(id: @details[:post_id])
       else
-        @topic.first_post
+        @topic&.first_post
       end
 
     return if post.blank?

spec/lib/composer_messages_finder_spec.rb

@@ -395,6 +395,11 @@ RSpec.describe ComposerMessagesFinder do
         )
       expect(finder.check_dont_feed_the_trolls).to be_present
     end
+
+    it "safely returns from not finding a post" do
+      finder = ComposerMessagesFinder.new(user, composer_action: "reply", topic_id: nil)
+      expect(finder.check_dont_feed_the_trolls).to be_blank
+    end
   end
 
   describe ".check_get_a_room" do