CKEditor image2 plugin breaks the CMS

Using the CKEditor image2 plugin to insert <img> elements can lead to Wicket component corruption and inability to save the document.

Reproduction

• 12.1.0 archetype, install Simple Content feature
• In Console, configure 'image2' plugin:
  /hippo:namespaces/hippostd/html/editor:templates/_default_/ckeditor.config.overlayed.json="{extraPlugins: 'image2'}"
• In CMS, use the image2 (second image icon) in a document, by trying to type an URL,
  e.g. http://www.demo.onehippo.com/images/gogreen-logo.png (copy-paste does work)

After having typed "http:", wait a bit, then this is visible in the Javascript Console:

wicket-ajax-jquery.min-ver-1510055568000.js:1 Wicket.Ajax:   Wicket.Ajax.Call.processComponent: Component with id [[idxxx]] was not found while trying to perform markup update. Make sure you called component.setOutputMarkupId(true) on the component whose markup you are trying to update.

At that point, the CMS page is corrupted. Either a Save & Close or some auto-reload leads to an almost white screen and "YAHOO is not defined" in Javascript Console.

Reproduced on Chrome 62 and Safari 11.

No markup saved in the draft.

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BTW, the fact that on every keystroke, a GET is sent for the URL that is typed, producing Javascript console errors continuously, is by design: at https://sdk.ckeditor.com/samples/captionedimage.html, double-click the rocket image and change the image URL to see 404s.