GtkBuilder F# Object reference not set to an instance of an object - f#

Question

I have a problem with making the mainwindow on my simple app run, The error given is that - Object reference not set to an instance of an object.
this happens when the app is getting debugged and the error occurs at handler.window1.ShowAll()
I did find some code online which hints at adding some member code as in member this.Whatever() = window1 however i have no idea if this is relevent to my code, or where to put it.
i am happy for any help you can give me as i have been trying all day to get this working in many ways and simply cannot.
namespace potato
module Main =
open System
open Gtk
type Handler()=class
[<Object>]
[<DefaultValue(true)>]
val mutable window1 : Window
end
[<EntryPoint>]
let Main(args) =
Application.Init()
let builder = new Builder("GUI.ui")
let handler = new Handler()
builder.Autoconnect(handler)
handler.window1.ShowAll()
Application.Run()
0
Here is the glade.xml
<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.20.0 -->
<interface>
<requires lib="gtk+" version="3.18"/>
<object class="GtkWindow" id="window1">
<property name="width_request">1024</property>
<property name="height_request">576</property>
<property name="can_focus">False</property>
<child>
<placeholder/>
</child>
</object>
</interface>

Answer 1

Right, the problem was right in front of my eyes, and i ended up having to go through old test projects to see and realise what #scrwtp was hinting at, this is the old working code fixed for gtk3 gtkbuilder.
namespace potato
module Main =
open System
open Gtk
type Handler()=class
[<Builder.Object>]
[<DefaultValue(true)>]
val mutable window1 : Window
end
let OnDelete (args:DeleteEventArgs) =
Application.Quit()
args.RetVal <- true
[<EntryPoint>]
let Main (args) =
Application.Init()
let gxml = new Builder("GUI.xml")
let handler = new Handler()
do gxml.Autoconnect(handler)
handler.window1.DeleteEvent
|> Event.add OnDelete
handler.window1.ShowAll()
Application.Run()
0
The reason, i now understand is that i had specified a handler and passed nothing to it, because nothing was passed it IE:(handler.window1.DeleteEvent) it simply wouldnt show when i called showall, hope this helps someone else with a similar problem