Fix [object object] for popovers

[jesperengstrom]

Solves

Checking that the _label is a string to prevent printing [object object], currently displaying in the role popovers.

example before / after

---

A bit more in-depth...
Data looking like:

"domain": {
   "@id": "https://id.kb.se/vocab/Work"
},
"_label": {
   "@id": "http://www.w3.org/2000/01/rdf-schema#domain",
   "labelByLang": {
      "en": "Domain",
      "sv": "Förekommer på"
   }
}

_label not being flattened to "Förekommer på", is that a bug? In the next recursion, domain content is hidden, since it contains an @id.

In any which case, I think the string check makes sense...

[johanbissemattsson]

LGTM

_label not being flattened to "Förekommer på", is that a bug? In the next recursion, domain content is hidden, since it contains an @id.

Yes, this sounds lika a bug in LXLJS and should be resolved there. I have created an issue for it: LWS-240.