related tags: fix exception on copyright tags with nonexistent wiki pages.

app/logical/related_tag_query.rb

@@ -48,7 +48,7 @@ class RelatedTagQuery
   def other_wiki_pages
     return [] unless Tag.category_for(query) == Tag.categories.copyright
 
-    other_wikis = DText.parse_wiki_titles(wiki_page.body).grep(/\Alist_of_/i)
+    other_wikis = DText.parse_wiki_titles(wiki_page&.body&.to_s).grep(/\Alist_of_/i)
     other_wikis = other_wikis.map { |name| WikiPage.titled(name).first }
     other_wikis = other_wikis.select { |wiki| wiki.tags.present? }
     other_wikis

test/functional/related_tags_controller_test.rb

@@ -3,7 +3,7 @@ require 'test_helper'
 class RelatedTagsControllerTest < ActionDispatch::IntegrationTest
   context "The related tags controller" do
     setup do
-      create(:post, tag_string: "touhou")
+      create(:post, tag_string: "copy:touhou")
     end
 
     context "show action" do
@@ -16,6 +16,11 @@ class RelatedTagsControllerTest < ActionDispatch::IntegrationTest
         get related_tag_path(format: :json), params: { query: "touhou" }
         assert_response :success
       end
+
+      should "work for .js responses for copyright tags without a wiki" do
+        get related_tag_path(format: :js), params: { query: "touhou" }, xhr: true
+        assert_response :success
+      end
     end
   end
 end