Elm 0.19 debugger crash when browser blocks popup

readme.md

Elm 0.19rc1 Browser.Debugger throws errors repeatedly when trying to open a popup if the browser (Chrome/Safari) is configured to block popups.

The problem is in _Debugger_openWindow on line 180. window.open() can return null for reasons, one of which being that the browser is blocking popups. On line 181 the value is used without checking for null.

Steps to reproduce:

1. Make sure that your browser is blocking popups:
   • on Safari, open Preferences -> Security -> check 'Block pop-up windows'
   • on Chrome, open Preferences -> search for "popups" -> click "Content Settings" -> click "Popups" -> adjust settings
2. elm make Main.elm --debug
3. open index.html
4. click "Explore History" in the corner
5. check the JS console to observes 100s/1000s of errors being logged

elm.json

{
    "type": "application",
    "source-directories": [
        "."
    ],
    "elm-version": "0.19.0",
    "dependencies": {
        "direct": {
            "elm/browser": "1.0.0",
            "elm/core": "1.0.0",
            "elm/html": "1.0.0"
        },
        "indirect": {
            "elm/json": "1.0.0",
            "elm/time": "1.0.0",
            "elm/url": "1.0.0",
            "elm/virtual-dom": "1.0.0"
        }
    },
    "test-dependencies": {
        "direct": {},
        "indirect": {}
    }
}

Main.elm

module Main exposing (..)

import Browser
import Html exposing (text)


main =
    Browser.sandbox
        { init = init
        , view = view
        , update = update
        }

init = ()

type Msg = NoOp

update msg model =
    case msg of
        NoOp ->
            model

view model =
    text "hi"