Partial SKOS XL label data breaks headline on concept page

[osma]

At which URL did you encounter the problem?

https://dev.finto.fi/yso/fi/page/p16810

(not visible right now as the offending SKOS XL data was removed)

What steps will reproduce the problem?

1. Add SKOS XL data for one of the prefLabels of a concept (e.g. Northern Sami prefLabel in this case)
2. Look at the concept page in a different content language (e.g. Finnish)

What is the expected output? What do you see instead?

Expected to see a normal headline showing the preferred label in the chosen content language. But instead the headline is missing:

(originally reported by @nikopartanen)

What browser did you use? (eg. Firefox, Chrome, Safari, Internet explorer)

Firefox