NullReferenceException when using razor if statements

I have a simple details view that uses two if statements if the user is a Customer, it displays the customer properties. If the user is an Investor it displays their properties instead.

My issue is that my if statement works for one or the other but not both. Giving me a:

NullReferenceException: Object reference not set to an instance of an object

when trying to use both if statements.

My view

@model MyProject.Models.ApplicationUser
<h3>
    @Html.DisplayFor(m => Model.FirstName)
    @Html.DisplayFor(m => Model.LastName)
</h3>
@if (Model.Customer.CustomerId != null)
{
    <div class="form-group">
        <strong>Tax ID:</strong>
        @Html.DisplayFor(m => m.Customer.TaxId)
    </div>
} else {
}

@if (Model.Investor.InvestorId != null)
{
    <div class="form-group">
        <strong>Social Security #:</strong>
        @Html.DisplayFor(m => m.Investor.SsnNum)
    </div>
    <div class="form-group"> 
        <strong>Date of birth:</strong>
        @Html.DisplayFor(m => m.Investor.DOB)
    </div>
} else {
}

Controller

public async Task<IActionResult> Details(string id)
{
    if (id == null || id.Trim().Length == 0)
    {
        return NotFound();
    }
    var userFromDb = await _db.ApplicationUser.Include(u => u.Investor).Include(u => u.Customer).FirstOrDefaultAsync(i => i.Id == id);
    if (userFromDb == null)
    {
        return NotFound();
    }
    return View(userFromDb);
}

Investor

public class Investor
{
    [Key, ForeignKey("ApplicationUser")]
    public string InvestorId { get; set; }
    public virtual ApplicationUser ApplicationUser { get; set; }

    [Required]
    [Display(Name = "SSN")]
    public string SsnNum { get; set; }
    [Display(Name = "Date of Birth")]
    public DateTime DOB { get; set; }
}