refedit will not save - missing application reference

I have opened a block to add a quick leader only and when I tried to close I got the message cannot save back ... (see screenshot 1) when I hit f2 the problem is shown to be a missing application reference (see screenshot 2).

This may not seem a big deal but the problem actually occurred after about 15mins of editing a block and it has taken me about an hour to figure out what the culprit was - any ideas on why this is happening and how to fix it?

Thanks

Oscar