Merge pull request #14129 from joankaradimov/fix-render-to-string

Fix `render_to_string` when called via manually instantiated controllers

actionpack/lib/abstract_controller/rendering.rb

@@ -106,7 +106,9 @@ def _process_format(format, options = {})
     def _normalize_render(*args, &block)
       options = _normalize_args(*args, &block)
       #TODO: remove defined? when we restore AP <=> AV dependency
-      options[:variant] = request.variant if defined?(request) && request.variant.present?
+      if defined?(request) && request && request.variant.present?
+        options[:variant] = request.variant
+      end
       _normalize_options(options)
       options
     end