FIX: Don't enqueue posts if the user can't create them (ex: closed)

lib/guardian/topic_guardian.rb

@@ -20,6 +20,7 @@ module TopicGuardian
 
   def can_create_post_on_topic?(topic)
     # No users can create posts on deleted topics
+    return false if topic.blank?
     return false if topic.trashed?
     return true if is_admin?
 

lib/new_post_manager.rb

@@ -80,6 +80,17 @@ class NewPostManager
   def self.default_handler(manager)
     if user_needs_approval?(manager)
 
+      # Can the user create the post in the first place?
+      if manager.args[:topic_id]
+        topic = Topic.unscoped.where(id: manager.args[:topic_id]).first
+
+        unless manager.user.guardian.can_create_post_on_topic?(topic)
+          result = NewPostResult.new(:created_post, false)
+          result.errors[:base] << I18n.t(:topic_not_found)
+          return result
+        end
+      end
+
       result = manager.enqueue('default')
 
       if is_fast_typer?(manager) || matches_auto_block_regex?(manager)