Add test for GetSerializer failure when serializers direct access is used. serializers is only used in a session and only exists after the first call.

Author: elachlan

src/System.Windows.Forms.Design/tests/UnitTests/System/ComponentModel/Design/Serialization/DesignerSerializationManagerTests.cs

@@ -1394,6 +1394,9 @@ public void DesignerSerializationManager_IDesignerSerializationManagerGetSeriali
 
             // Call again.
             Assert.Null(iManager.GetSerializer(objectType, typeof(int)));
+
+            // Call unrelated object type
+            Assert.Null(iManager.GetSerializer(typeof(object), typeof(int)));
         }
 
         [Fact]