I'm experiencing Bug BUG-000094906 and I wonder if there's a fix for this already?
AGOL Application Search Widget generates "no results" message
As you can see on bug status page, it is not currently in the product plan. In the meantime, you can work around this issue by changing the maximum number of suggestions for the Search widget in WAB to "0". Now, Search widget only shows results (not suggestions) and you won't get "no results" error anymore.
All the best,
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