As revealed by request #40080, given a cross-tracker search widget that was saved in Expert mode, when I switch it from Expert to Default mode (uncheck the switch), and I select at least one tracker BUT I do not change the query (I leave an expert query with SELECT ...)
then there is the following JavaScript error in the console, and the widget appears "blocked", it does not show an error feedback but the Search button is disabled.

cross-tracker-BvKKMYVf.js:1 TypeError: Cannot read properties of undefined (reading 'length')

This happens because we intentionally throw an empty SyntaxError in some cases, which has its line number to zero, which causes a problem in the code trying to show where the syntax error comes from in the query. Instead of "hacking" the SyntaxError, we should use another kind of error message, since we trigger this error outside of a parser, we have no idea how the initial query is written. We should not try to guess, we can instead tell the user that the query is invalid for the specific case we encounter.