Exception after changing a route while using CKEditorContext

[Mgsy]

Steps to reproduce

Generic steps to reproduce

1. Create an app with <CKEditorContext /> and two routes - one for the sample and the second one for a blank HTML page.
2. After initializing the editor, change the route.

Scenario with demo

1. Download the demo project (real-time-collaboration-for-react.zip) and run it.
2. After initializing the editor, click on the Page 2 button.

Current result

There are errors in the console:

index.js:5 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'then')
    at index.js:5:167077
    at new Promise (<anonymous>)
    at sn._destroyEditor (index.js:5:167023)
    at sn.componentWillUnmount (index.js:5:165891)
    at pc (index.js:5:963026)
    at hc (index.js:5:962117)
    at pc (index.js:5:963200)
    at hc (index.js:5:962117)
    at pc (index.js:5:963068)
    at hc (index.js:5:962117)
    
index.js:5 Uncaught (in promise) TypeError: Cannot read properties of null (reading 'model')
    at index.js:5:159067

Notes

Demo anatomy:

src/sample.js - routing definition
src/app.js - the application code

I couldn't reproduce the issue without <CKEditorContext> component.

• "@ckeditor/ckeditor5-react": "5.0.3"
• "react": "^18.2.0",
• "react-dom": "^18.2.0",
• "react-router-dom": "^6.4.3",

[pomek]

Debug notes:

• We have two errors: the first comes from the react component, and the second reports ContextWatchdog (ckeditor5-watchdog).
  1. A promise returned by ContextWatchdog#remove() is ignored.

     ckeditor5-react/src/ckeditor.jsx

     Lines 353 to 355 in 4d433c5

     	destroy() {
     	this._contextWatchdog.remove( this._id );
     	}

     
     Adding return helps.
  2. https://github.com/ckeditor/ckeditor5/blob/0a3431e9828b291ce34053bef471fd1d13b23738/packages/ckeditor5-watchdog/src/editorwatchdog.js#L241
     EditorWatchdog tries to detach events from an editor that does not exist anymore.
• Let's focus on the second error.
• We do not have to detach events manually (https://github.com/ckeditor/ckeditor5/blob/0a3431e9828b291ce34053bef471fd1d13b23738/packages/ckeditor5-watchdog/src/editorwatchdog.js#L241), as in the following chain: editor > model > document, it is handled in a destroy() method.
• Then, for some reason (I don't know why yet), the destructor is a called with null instead of an editor instance.

To fix the problem, we need to:

• ckeditor5-react: do not ignore a promise,
• ckeditor5-watchdog: do not destroy a destroyed editor (it can be tricky to cover with a unit test).

[pomek]

Proposal: https://github.com/ckeditor/ckeditor5-react/compare/i/349?expand=1