Mention in the initial content throws error

[Mgsy]

Steps to reproduce

1. Open a sample containing a mention in the initial content (e.g. mentions manual tests).

Current result

The error is thrown

Error

mathtype.js:75 TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'writer' -> object with constructor 'Writer'
    |     property 'model' -> object with constructor 'Model'
    |     ...
    |     property 'attributes' -> object with constructor 'Object'
    --- property 'mention' closes the circle
    at JSON.stringify (<anonymous>)
    at ce (inspector.js:2)
    at inspector.js:2
    at Array.map (<anonymous>)
    at be (inspector.js:2)
    at inspector.js:2
    at ge (inspector.js:2)
    at inspector.js:2
    at ge (inspector.js:2)
    at inspector.js:2

Notes

It works fine if we load the data without a mention, insert a mention and call editor.setData( editor.getData() ).

[oleq]

https://github.com/ckeditor/ckeditor5-inspector/blob/master/src/components/utils.js#L15 is the root of the problem. The question remains what data causes this as it gets stringified.