I have two tables Category and Document. See relationships in picture See picture

I wrote the following query to select data from both tables based on relationship

public List<DocumentViewModel> All()
        {
            var docs  = _context.Document.ToList();
            List<DocumentViewModel> docList = docs.Select(x => new DocumentViewModel
            { DocumentId = x.DocumentId,
              DocumentPath = x.DocumentPath,
              CategoryId = x.CategoryId,
              CategoryName = x.Category.CategoryName }).ToList();
            return docList;

        }

when this function is called , I get the following error

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Here are my modals

document

public class Document
    {
        [Key]
        public int DocumentId { get; set; }
        [Required]
        public string DocumentPath { get; set; }
        public Nullable<int> CategoryId { get; set; }
        public virtual Category Category { get; set; }
    }

Category

public class Category
    {

        [Key]
        public int CategoryId { get; set; }
        [Required]
        public string CategoryName { get; set; }
        public virtual ICollection<Document> Documents { get; set; }
    }

DocumentViewModel

 public class DocumentViewModel
    {
        public int DocumentId { get; set; }
        public string DocumentPath { get; set; }
        public int? CategoryId { get; set; }
        public string CategoryName { get; set; }
    }

Any Idea where am doing mistake?