DataSource Controls :: Receiving Error - System.NullReferenceException - Object Reference Not Set

I receive the following error in the code below: System.NullReferenceException: Object reference not set to an instance of an object. When I am logged into the page I do not receive the error. My Code:

Partial Class _Default
Inherits System.Web.UI.Page
Protected Sub SqlDataSource4_Selecting(ByVal sender As Object, ByVal e As SqlDataSourceCommandEventArgs) Handles SqlDataSource4.Selecting
Dim MemUser As MembershipUser
MemUser = Membership.GetUser()
UserID.Text = MemUser.ProviderUserKey.ToString()
e.Command.Parameters("@UserId").Value = MemUser.ProviderUserKey.ToString()
End Sub
End Class

I read the following online: You are trying to use a reference variable who's value is Nothing/null. When the value is Nothing/null for the reference variable, that means it is not actually holding a reference to an instance of any object that exists on the heap. You either never assigned something to the variable, never created an instance of the value assigned to the variable, or you set the variable equal to Nothing/null manually, or you called a function that set the variableto Nothing/null for you. The following is the Arthur's code example and his suggested fix which I am sure is right:

Private Sub TestString()
Dim a As String
If a.Length = 0 Then
Console.Write("Yes")
End If
End Sub