Erased union pattern matching fails on a union with a [<Constant null>] case

[Tarmil]

The following:

type MyType =
    | [<Constant(null)>] NullCase
    | NonNullCase

let f (x: Union<Dom.Node, MyType>) =
    match x with
    | Union1Of2 _ -> "it's not my type"
    | Union2Of2 _ -> "it's my type"

compiles to this:

f=function(x)
 {
  return x.constructor===Global.Object?"it's my type":"it's not my type";
 };

The problem is that if we call f (Union2Of2 NullCase), then x is null and the access to .constructor fails.

This issue causes crashes in WebSharper.UI here.

[Tarmil]

Fixed the example -- this happens with Union<_, MyType>, not with Union<MyType, _> as I originally posted.