Web Inspector: Uncaught Exception: undefined is not an object (evaluating 'console.assert(listenersForEventType, this, eventType, listener, thisObject)')

[Devin Rousso]

Uncaught Exception in Web Inspector.

Steps to Reproduce:
0. make sure the Sources Tab is selected before Web Inspector is opened
1. inspect <https://devinrousso.com/demo/WebKit/bipbop-cc-subtitles-chapters.html>
2. switch to the Elements Tab

Uncaught Exceptions:
-----------------------
 - undefined is not an object (evaluating 'console.assert(listenersForEventType, this, eventType, listener, thisObject)') (at Object.js:84:23)
    removeEventListener @ Object.js:84:23
    ondetach @ DOMTreeElement.js:462:51
    _detach @ TreeElement.js:294:26
    removeChildAtIndex @ TreeOutline.js:341:22
    _updateChildren @ DOMTreeElement.js:558:36
    _updateChildren @ [native code]
    mycallback @ DOMNode.js:604:25
    mycallback @ [native code]
    _dispatchResponseToCallback @ Connection.js:152:27
    _dispatchResponse @ Connection.js:122:45
    dispatch @ Connection.js:77:35
    dispatchMessageFromTarget @ TargetManager.js:176:39
    dispatchMessageFromTarget @ TargetObserver.js:47:51
    _dispatchEvent @ Connection.js:210:26
    dispatch @ Connection.js:79:32
    dispatch @ InspectorBackend.js:232:52
    ? @ MessageDispatcher.js:42:34

Additional Details:
    cause --> An uncaught exception was thrown while dispatching response callback for command DOM.requestChildNodes.
-----------------------

Notes:
Inspected URL:        https://devinrousso.com/demo/WebKit/bipbop-cc-subtitles-chapters.html
Loading completed:    true
Frontend User Agent:  Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko)
Dispatch Source:      Protocol Event

Protocol Event:
{"method":"Target.dispatchMessageFromTarget","params":{"targetId":"page-23","message":"{\"result\":{},\"id\":215}"}}
Dispatch Source:      Protocol Command Response

Protocol Command Response:
{"result":{},"id":215}

Protocol Command Request:
{"id":215,"method":"DOM.requestChildNodes","params":{"nodeId":47}}

[Radar WebKit Bug Importer]

<rdar://problem/76223103>