AjaxObservable: this / xhr is of type null

[freiondrej]

Bug Report

Current Behavior
In some circumstances the ajax observables throw an internal error when xhr is of type null (AjaxObservable.js:340) or this is of type null (AjaxObservable.js:278). (Possibly related to #4095 ?)

I'm using webpack so here is more description of which code parts are failing in case the line numbers are confused:

xhr being of type null:
this.status = xhr.status; from block function AjaxErrorImpl(message, xhr, request) {
this being of type null:
if (this.readyState === 4) { from block function xhrLoad(e) {

Reproduction

import { of, timer } from 'rxjs';
import { switchMap } from 'rxjs/operators';
import {ajaxGet} from "rxjs/internal-compatibility";


const source = of('').pipe(switchMap(_ => ajaxGet('http://example.org')))

source.subscribe(x => console.log(x)); // ok

const source2 = timer(0).pipe(switchMap(_ => ajaxGet('http://example.org')))

source2.subscribe(x => console.log(x)); // also ok
const source3 = timer(1000).pipe(switchMap(_ => ajaxGet('http://example.org')))

source3.subscribe(x => console.log(x)); // not ok

Expected behavior
I expect the internal code not to fail this way - either produce a more friendly error if I'm doing something wrong or succeed without error.

Environment

• Runtime: node 13.2.0, Firefox 76.0 / Safari 13.1
• RxJS version: 6.5.4
• webpack 4.42.0