How to read from a file an 2d array ? vb.net

I'm new in vb.net programming, and i want to read a 2d array from a file. I searched a lot and i can't figure out how can i do that. There is the input file :

1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1

And here is the code part :

Dim map As Integer(,)
Dim reader As StreamReader
    reader = IO.File.OpenText(folder + "\harta\harta.txt")
    Dim linie As String, i, j As Integer
    For i = 0 To 10
        For j = 0 To 12
            linie = reader.ReadLine()
            map(i, j) = linie.Substring(j, linie.IndexOf(" ")) 'here is my problem'
        Next j
    Next i
    reader.Close()

When i run the code, i get the following error:

An unhandled exception of type 'System.NullReferenceException' occurred in WindowsApplication1.exe