Protect against missing parents

changelog/fix_missing_parents_in_duplicated_test_run_cop.md

@@ -0,0 +1 @@
+* [#167](https://github.com/rubocop/rubocop-minitest/pull/167): Fix potential for valid Ruby code to be unparsable in `Minitest/DuplicateTestRun` cop. ([@gjtorikian][])

lib/rubocop/cop/minitest/duplicate_test_run.rb

@@ -63,7 +63,10 @@ def on_class(class_node)
 
         def parent_class_has_test_methods?(class_node)
           parent_class = class_node.parent_class
-          parent_class_node = class_node.parent.each_child_node(:class).detect do |klass|
+
+          return false unless (class_node_parent = class_node.parent)
+
+          parent_class_node = class_node_parent.each_child_node(:class).detect do |klass|
             klass.identifier == parent_class
           end
 

test/rubocop/cop/minitest/duplicate_test_run_test.rb

@@ -103,4 +103,17 @@ def test_child_asserts_twice
       end
     RUBY
   end
+
+  def test_does_not_throw_error_if_missing_parent_test_class
+    assert_no_offenses(<<~RUBY)
+      class PostmarkAccountServiceTest < ActionDispatch::IntegrationTest
+        test "it handles missing payloads from Postmark Account API errors" do
+          Postmark::AccountApiClient.any_instance.stubs(:create_server).raises(StandardError)
+
+          Rails.logger.expects(:error)
+          PostmarkAccountService.create_server({})
+        end
+      end
+    RUBY
+  end
 end