NOJIRA Make sure we don't call method on non-object
PROV-912 Add-to-each mode for preflabels in batch editor

To properly support this, we have to make sure

a) that we remove [BLANK] labels before we add new ones for the
respective locale

and

b) that we don’t add multiple preferred labels for one locale, which is
certainly possible using the API but shouldn’t be via UI