private tiles not visible in editor

[iham]

to reproduce:

create a page and publish it
create a page and don't publish it

create a page and switch to mosaic view and hit "edit"
now customise the mosaic layout and add a "existing content" tile
navigate to the published page object and hit save.
all is fine
doing the same (add existing content) with a private page object will throw an error:

WARNING plone.jsonserializer Constraint not satisfied for value "80f986547d504295b2ef24309b338457" of field "content_uid". Returning None instead.
WARNING plone.jsonserializer Deserializer not found for value "80f986547d504295b2ef24309b338457" of field "content_uid". Returning None instead.

don't mind the uid in particular; see it as a sample.
no content of that page is shown in the mosaic editor but after saving, that content is visible.

[petschki]

FYI: solution is underway and works already on Py2 ... fighting with a Py3 issue though

plone/plone.app.standardtiles#101