Fix undefined method uncached for polymorphic belongs_to #20426

Unitialized polymorphic `belongs_to` associations raise an error while
attempting to reload, as they attempt to make an uncached reload, but
don't have a klass to fetch uncachedly. In this case, `loaded?` should
be `false` anyway.

activerecord/lib/active_record/associations/singular_association.rb

@@ -3,7 +3,7 @@ module Associations
     class SingularAssociation < Association #:nodoc:
       # Implements the reader method, e.g. foo.bar for Foo.has_one :bar
       def reader(force_reload = false)
-        if force_reload
+        if force_reload && klass
           klass.uncached { reload }
         elsif !loaded? || stale_target?
           reload

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -288,6 +288,7 @@ def test_with_condition
   def test_polymorphic_association_class
     sponsor = Sponsor.new
     assert_nil sponsor.association(:sponsorable).send(:klass)
+    assert_nil sponsor.sponsorable(force_reload: true)
 
     sponsor.sponsorable_type = '' # the column doesn't have to be declared NOT NULL
     assert_nil sponsor.association(:sponsorable).send(:klass)