Under some circumstances, an error is caused in the front-end code that causes freezing the UI when saving a list of snaks. A timing issue seems to be very likely.
This was experienced on wikidata.org adding a seventh snak to a list of reference snaks on a rather small item. I was able to reproduce the error locally adding a 4th snak to a set of reference snaks on a rather huge item and adding a 9th snak on a very small item. Consequently, this might correlate with the overall item size in some way.
The error occurs in wikibase.SnakList.add, the function receives "null" instead of a SnakList object. Apart from solving the actual issue, the mechanism should be more robust since snaklistview.value() that is used to generate the parameter may indeed return "null".

Version: unspecified
Severity: normal
Whiteboard: u=dev c=frontend p=13