On https://www.wikidata.org/wiki/Q42, I get:
jQuery.Deferred exception: this is undefined SELF.prototype._buildReport@https://www.wikidata.org/w/extensions/WikibaseQualityConstraints/modules/gadget.js?63f1d:285:33 SELF.prototype._addResultsToSnak@https://www.wikidata.org/w/extensions/WikibaseQualityConstraints/modules/gadget.js?63f1d:434:17 SELF.prototype._addReportsToStatement@https://www.wikidata.org/w/extensions/WikibaseQualityConstraints/modules/gadget.js?63f1d:495:18 SELF.prototype._renderWbcheckconstraintsResult/<@https://www.wikidata.org/w/extensions/WikibaseQualityConstraints/modules/gadget.js?63f1d:117:6 each@https://www.wikidata.org/w/resources/lib/jquery/jquery.js?6a07d:354:10 each@https://www.wikidata.org/w/resources/lib/jquery/jquery.js?6a07d:189:10 SELF.prototype._renderWbcheckconstraintsResult@https://www.wikidata.org/w/extensions/WikibaseQualityConstraints/modules/gadget.js?63f1d:111:5 mightThrow@https://www.wikidata.org/w/resources/lib/jquery/jquery.js?6a07d:3534:21 resolve/</process<@https://www.wikidata.org/w/resources/lib/jquery/jquery.js?6a07d:3602:12
Happens in safemode=1.
Another item I could reproduce this on is https://www.wikidata.org/wiki/Q1.