Object reference error when saving content

Description

Steps to reproduce:

1. On an Alloy site, create a property in an EditorialBlock, as below:

[Display(
 GroupName = SystemTabNames.Content,
 Order = 320)]
 [MaxLength(10)]
 public virtual string MyString { get; set; }

1. In admin view, add a property named EditBlock1 with type EditorialBlock to News Page:
   
2. In edit view, create a new page with type News Page, edit the Main Body property of EditorialBlock (for example, set the Main Body text to 'Test') then publish the page.
3. Set the MyString property of the EditorialBlock as [Required].
4. Dynamically update content of the page as following code:

         var contentRepository = ServiceLocator.Current.GetInstance <IContentRepository>();
         var content = contentRepository.Get<NewsPage>(new ContentReference(113)); // assume that the newly created page has ID 113
         var c = content.CreateWritableClone();
         c.Property["EditBlock1"].Clear();
         contentRepository.Save(c); 

Expected result:

Content is updated successfully.

Actual result:

Exception `Object reference not set to an instance of an object` is thrown.