FIX: error when deleting a tag associated with a deleted topic

app/models/topic_tag.rb

@@ -3,7 +3,7 @@ class TopicTag < ActiveRecord::Base
   belongs_to :tag, counter_cache: "topic_count"
 
   after_create do
-    if topic.category_id
+    if topic&.category_id
       if stat = CategoryTagStat.where(tag_id: tag_id, category_id: topic.category_id).first
         stat.increment!(:topic_count)
       else
@@ -13,7 +13,7 @@ class TopicTag < ActiveRecord::Base
   end
 
   after_destroy do
-    if topic.category_id
+    if topic&.category_id
       if stat = CategoryTagStat.where(tag_id: tag_id, category: topic.category_id).first
         stat.topic_count == 1 ? stat.destroy : stat.decrement!(:topic_count)
       end

spec/models/tag_spec.rb

@@ -15,6 +15,13 @@ describe Tag do
     SiteSetting.min_trust_level_to_tag_topics = 0
   end
 
+  it "can delete tags on deleted topics" do
+    tag = Fabricate(:tag)
+    topic = Fabricate(:topic, tags: [tag])
+    topic.trash!
+    expect { tag.destroy }.to change { Tag.count }.by(-1)
+  end
+
   describe '#tags_by_count_query' do
     it "returns empty hash if nothing is tagged" do
       expect(described_class.tags_by_count_query.count(Tag::COUNT_ARG)).to eq({})