vs2017 Wait for WPF to finish?

[pmeloy]

There isn't much to explain. I have this button

<Button x:Name="btnAdd" Content="Add" Grid.Row="1" Grid.Column="2" IsEnabled="False"/>

Then in a combobox event I call this sub and I try to set it enabled or disabled

    Private Sub EnableAdd()
        If loading = False Then
            If tbxName.Text <> "" And tbxServing.Text <> "" Then
                btnAdd.IsEnabled = True
            Else
                btnAdd.IsEnabled = False
            End If
        End If
    End Sub

When I open the form it immediately says an object is not set to an instance of an object. If I comment out the isenabled lines it loads fine