Editable marker throws TypeError: Cannot read properties of undefined

[seraphr]

steps to reproduce

1. open Editable marker example
2. open developer tool and select Console tab
3. click This is editable marker
4. error is displayed in console
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     • vis-timeline/lib/timeline/component/CustomTime.js

       Lines 218 to 220 in 675ae8d

       	this.marker.addEventListener('pointerdown', function () {
       	this.marker.focus();
       	});