LEAP Panel sometimes becomes empty

[KasraF]

From Nadia:

One tiny bug I encountered is that sometimes when I tried synthesizing, the panel appears but there's nothing in it.
But I was just able to esc from it and try again, so that wasn't a big issue

[KasraF]

I could only reproduce this problem in the case where the model returned all-empty completions. I added a special-case error message for that in 7cda126, but let's keep this issue open for now in case we run into this issue again?