Fix for a Catalog UI issue in EPiServer Commerce 8.7.1

In result of an upgrade to EPiServer Commerce 8.7.1, We were having an issue On Adding Media Item for a product/variant from Online Centre - Catalog UI. Details of exception can be found at the end of article. If you are not ready to upgrade to EPiServer Commerce 8.8.0 yet, just upgrade EPiServer.CMS.UI to 7.19.1. It is safe to upgrade this version as EPiServer.CMS.UI 7.19.1 is the version released just after EPiServer Commerce 8.7.1 and Core 7.19.2, therefor it has tested for compatibility.

Excepton Details are following
2015-02-24 13:11:09,333 [13] ERROR EPiServer.Cms.Shell.UI.Rest.ContentChangeManager - Unexpected error when updating property commerceMediaCollection
System.ArgumentNullException: Value cannot be null.
Parameter name: url
   at EPiServer.UrlBuilder..ctor(String url)
   at EPiServer.Commerce.Shell.ObjectEditing.InternalMetadata.CommerceMediaModelConverter.ToServerModel(Object clientModel)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at EPiServer.Commerce.Serialization.Json.ItemCollectionConverter.ReadJson(JsonReader reader, Type objectType, Object existingValue, JsonSerializer serializer)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at EPiServer.Cms.Shell.UI.Rest.ContentChangeManager.Deserialize(String value, Type type)
   at EPiServer.Cms.Shell.UI.Rest.ContentChangeManager.UpdateProperties(IDictionary`2 properties, IContent content)
System.ArgumentNullException: Value cannot be null.
Parameter name: url
   at EPiServer.UrlBuilder..ctor(String url)
   at EPiServer.Commerce.Shell.ObjectEditing.InternalMetadata.CommerceMediaModelConverter.ToServerModel(Object clientModel)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at EPiServer.Commerce.Serialization.Json.ItemCollectionConverter.ReadJson(JsonReader reader, Type objectType, Object existingValue, JsonSerializer serializer)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at EPiServer.Cms.Shell.UI.Rest.ContentChangeManager.Deserialize(String value, Type type)
   at EPiServer.Cms.Shell.UI.Rest.ContentChangeManager.UpdateProperties(IDictionary`2 properties, IContent content)