ASP.NET Core Object Reference not set to an instance for foreign key search

I have these queries. The first initialData gets a list of Ids without a filter and the second query filters out the results based on user inputs. FieldId and CategoryId are foreign/primary keys of their own table. The issue is when the application first loads, nulls are sent through and thats acceptable. However when it reaches the second query, I get the Object Reference not set to an instance error.

Is there such thing as a null text identifier in .net core/c#?

var initialData = (from item in dbContext.DocumentCategories
    join df in dbContext.DocumentFields
    on item.Id equals df.DocumentCategoriesId
    join dfs in dbContext.DocumentFieldsStore
    on df.Id equals dfs.DocumentFieldsId
    select new SearchDocumentsListViewModel
    {
        CategoryId = item.Id,
        DocumentId = dfs.DocumentsId,
        FieldId = df.Id
    })
    .ToList();

initialData = initialData
.Where(u => u.CategoryId.Contains(CategoryId) &&
u.FieldId.Contains(FieldId) &&
u.Data.Contains(FilterInput.Data))
.ToList();

I have also discovered that when I put "0" as the value, the same error triggers but if I put something like "abc", it goes on fine.

NOTE: I use GUIDs for my Ids