initialization of fsharp class field
===

I have a class bound to a XAML file.
Upon the first call to it, the field selectedudl is = null.

Do you know why it is the case ?
I would have expected it to be = "" ..





    type QuotesViewModel() =
        inherit ViewModelBase()
        let mutable histo = Array.empty
        let mutable selectedudl = ""
    
    
        member x.GetUnderlyings = seq { yield "one"; yield "two"}
    
        member x.SelectedUdl 
          with get() = selectedudl
          and set value = 
                   selectedudl  <- value
                   x.OnPropertyChanged <@ x.SelectedUdl @>
                   x.OnPropertyChanged <@ x.SelectedGraph @>
    
        member x.SelectedGraph 
          with get() = 
                   let graph  = asyncquotes.visualizehistoForUdl  selectedudl
                   let host =  new System.Windows.Forms.Integration.WindowsFormsHost()
                   host.Child <- new ChartControl(graph, Dock = DockStyle.Fill)
                   host

