Two wpf forms communicating with vb.net

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VB

WPF

I have been following the tutorial linked below to get an understanding of how to communicate from one form to another using vb.net.
http://www.developerfusion.com/samplechapter/4375/building-windows-applications
For my test application I have a wpf form with an image control and a button control. The image source is set by a 'Loaded' event handler and the button opens another form. On the second form is only a button. When the button's click event is raised I want the image in the first for to change.

I imagine that I am overlooking something simple but here is non-working code so far:

Class MainWindow 
    Public changer As New Window1
    Private Sub Grid1_Loaded(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles Grid1.Loaded
        Dim location As String = "pack://application:,,,/movie_ages.png"
        Dim imageresource As New BitmapImage
        imageresource.BeginInit()
        imageresource.UriSource = New Uri(location)
        imageresource.EndInit()
        Image1.Source = imageresource
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles Button1.Click
        If changer Is Nothing Then
            changer = New Window1
            changer.caller = Me
        End If
        changer.Show()
    End Sub
End Class

Public Class Window1
    Public caller As MainWindow
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles Button1.Click
        Dim location As String = "pack://application:,,,/Tools.png"
        Dim imageresource As New BitmapImage
        imageresource.BeginInit()
        imageresource.UriSource = New Uri(location)
        imageresource.EndInit()
            caller.Image1.Source = imageresource
    End Sub
End Class

When I run this code I get an error at caller.Image1.Source = imageresource in Pubic Class Window1 stating 'NullReferenceException was unhandled'

Any help would be greatly appreciated.

Thank you,
Jason Self