Getting a NullReferenceException when adding something to a list - vb.net

Question

I get a null reference exception when I try to use this webservice I'm working on. I have two fields in the object ipadarticle called fullname and tags, which are declared to be lists, so that ipadarticle can return multiple tags and authors. The null reference exception points to
ipadarticle2.FullName.Add(a_var.firstname + " " + a_var.lastname)
ipadarticle2.Tag.Add(a_var.tagtext)
I'm pretty new to vb programming, so I'm not really to sure what is causing this. To clarify, what is going on is that this stored procedure is fetching entries from a db, which has a list of articles with -among other things- tags and authors associated with it. Since articles have multiple tags and authors there are multiple entries for each article. When I return the info in the web service I am trying to make it so that only one ipadarticle object is returned for reach article, and then that contains a list of the multiple tags and authors associated with each article. I'm having a headache trying to figure this out.
Dim lq As New lqDFDataContext
Dim var = lq.mobile_IpadGetSavedArticlesAR(simpuser.UserID, tempParamKW(0), tempParamKW(1), tempParamKW(2), tempParamKW(3), tempParamKW(4), pageNum, pageLen)
Dim ipadarticle2 As New ipadArticle()
For Each a_var In var
If a_var.articleID <> temp Then
If flag = 0 Then
result.add(ipadarticle2)
Dim ipadarticle1 As New ipadArticle()
ipadarticle2 = ipadarticle1
End If
ipadarticle2.ArticleID = a_var.articleID
ipadarticle2.PublishedOn = a_var.publicationdate
ipadarticle2.Title = a_var.title
ipadarticle2.MedAbbr = a_var.medabbr.Replace(" ", "-").ToLower()
ipadarticle2.FullName.Add(a_var.firstname + " " + a_var.lastname)
ipadarticle2.Tag.Add(a_var.tagtext)
flag = 1
Else
ipadarticle2.Tag.Add(a_var.tagtext)
ipadarticle2.FullName.Add(a_var.firstname + " " + a_var.lastname)
flag = 0
End If
temp = a_var.articleID
Next
End If
Return result
ipadArticle class:
Imports Microsoft.VisualBasic
Public Class ipadArticle
Inherits SimpleObject
Public Sub New()
End Sub
Private _ArticleID As Integer
Public Property ArticleID() As Integer
Get
Return _ArticleID
End Get
Set(ByVal value As Integer)
_ArticleID = value
End Set
End Property
Private _Title As String
Public Property Title() As String
Get
Return _Title
End Get
Set(ByVal value As String)
_Title = value
End Set
End Property
Private _PublishedOn As String
Public Property PublishedOn() As String
Get
Return _PublishedOn
End Get
Set(ByVal value As String)
_PublishedOn = value
End Set
End Property
Private _MedAbbr As String
Public Property MedAbbr() As String
Get
Return _MedAbbr
End Get
Set(ByVal value As String)
_MedAbbr = value
End Set
End Property
Private _Tag As List(Of String)
Public Property Tag() As List(Of String)
Get
Return _Tag
End Get
Set(ByVal value As List(Of String))
_Tag = value
End Set
End Property
Private _FullName As List(Of String)
Public Property FullName() As List(Of String)
Get
Return _FullName
End Get
Set(ByVal value As List(Of String))
_FullName = value
End Set
End Property
End Class

Answer 1

The most likely cause is that the objects FullName and Tag have not been created (are Nothing) in ipadarticle2. These should most likely be created as new objects in the class constructor.
EDIT:
Based on the posted class, the above assumption was correct: FullName and Tag are defined as List(Of String), but the backing members are never created.
This can be fixed in a couple of ways:
1) Instantiate the backing member variables directly in their definition, i.e.:
Private _FullName As New List(Of String)
2) Instantiate the backing member variables in the constructor:
Public Sub New()
_FullName = New List(Of String)
_Tag = New List(Of String)
End Sub
3) Instantiate the backing member variable in the getter if it is nothing:
Public Property Tag() As List(Of String)
Get
If _Tag Is Nothing Then
_Tag = New List(Of String)
End If
Return _Tag
End Get
Basically, any variable types other than simple data types must be instantiated before they can be used (unless you test them for Nothingness).