Bug Report

Current Behavior
Calling .flush() on a scheduler (e.g. animationFrameScheduler) with no pending actions causes an unclear exception to be thrown:

TypeError: Cannot read property 'execute' of undefined
at AnimationFrameScheduler.flush (https://rxjs-pc1ga8.stackblitz.io/turbo_modules/rxjs@6.4.0/internal/scheduler/AnimationFrameScheduler.js:31:32)
at Object.eval (https://rxjs-pc1ga8.stackblitz.io/~/index.ts:12:36)
at eval (https://rxjs-pc1ga8.stackblitz.io/~/index.ts:19:4)
at eval (https://rxjs-pc1ga8.stackblitz.io/~/index.ts:20:3)
at eval (<anonymous>)
at Qt (https://c.staticblitz.com/d/webcontainer.e1e926a8972546110c3.js:15:30145)
at https://c.staticblitz.com/d/webcontainer.e1e926a8972546110c3.js:15:38799
at U (https://c.staticblitz.com/d/webcontainer.e1e926a8972546110c3.js:15:13565)
at https://c.staticblitz.com/d/webcontainer.e1e926a8972546110c3.js:15:13207
at boot (https://rxjs-pc1ga8.stackblitz.io/dev/boot:6:3)

Reproduction

• REPL or Repo link: https://stackblitz.com/edit/rxjs-pc1ga8?devtoolsheight=60

import {animationFrameScheduler} from 'rxjs';
animationFrameScheduler.flush();

Expected behavior
Either one of:

• not throwing when there are no pending actions
• throwing a clearer error (e.g. 'Attempting to flush an empty queue')

Environment

• Runtime: Chrome v73
• RxJS version: 6.4.x

Possible Solution

See expected behavior. This appears to affect all schedulers that have this do while loop without verifying that a valid action has been shift'd off of this.actions

This may be working as intended (and no change desired for performance reasons) in which case just having this issue open and google-able would provide additional documentation

Additional context/Screenshots

#2697 Appears to have been the same issue given their stacktrace: https://github.com/ReactiveX/rxjs/blob/5.4.1/src/scheduler/AsapScheduler.ts#L17

A clearer error (or no error at all) would probably have pointed them in the right direction.

Happy to submit a fix myself if the desired behavior is defined.