FIX: Include `assigned_messages_count` only when viewing assigned list.

plugin.rb

@@ -162,8 +162,10 @@ after_initialize do
   add_to_serializer(:topic_list, 'include_assigned_messages_count?') do
     options = object.instance_variable_get(:@opts)
 
-    scope.is_staff? ||
-      options.dig(:assigned)&.downcase == scope.current_user&.username_lower
+    if assigned_user = options.dig(:assigned)
+      scope.is_staff? ||
+        assigned_user.downcase == scope.current_user&.username_lower
+    end
   end
 
   add_to_serializer(:topic_view, :assigned_to_user, false) do

spec/serializers/topic_list_serializer_spec.rb

@@ -45,7 +45,40 @@ RSpec.describe TopicListSerializer do
         .to eq(1)
     end
 
+    describe 'when not viewing assigned list' do
+      let(:topic_list) do
+        TopicQuery.new(user).list_private_messages_assigned(user)
+      end
+
+      describe 'as an admin user' do
+        let(:guardian) { Guardian.new(Fabricate(:admin)) }
+
+        it 'should not include the attribute' do
+          expect(serializer.as_json[:topic_list][:assigned_messages_count])
+            .to eq(nil)
+        end
+      end
+
+      describe 'as an anon user' do
+        let(:guardian) { Guardian.new }
+
+        it 'should not include the attribute' do
+          expect(serializer.as_json[:topic_list][:assigned_messages_count])
+            .to eq(nil)
+        end
+      end
+    end
+
     describe 'viewing another user' do
+      describe 'as an anon user' do
+        let(:guardian) { Guardian.new }
+
+        it 'should not include the attribute' do
+          expect(serializer.as_json[:topic_list][:assigned_messages_count])
+            .to eq(nil)
+        end
+      end
+
       describe 'as a staff' do
         let(:guardian) { Guardian.new(Fabricate(:admin)) }