searching stops working completely

Sometimes searching in Geary stops working completely: no matter what I type, the spinner in the conversation pane spins forever and no search results are displayed at all. When this happens, the only fix is to restart Geary.

To be clear, this is not the same behavior as #474 (closed) (search resuts with "from:" are sometimes incomplete), and so I'm opening a separate bug. In #474 (closed), some search results are displayed, but some results that should be there are missing, and the displayed results still change as I type or delete characters in the search box. When this bug occurs, no results appear at all, no matter what I type, even if I erase all characters in the search box.

By the way this is account-specific: if this problem occurs in one account, searching may still work in another account.

I probably see this happen once per day or so.