First: report this to Bubble support. It’s probably a bug (I’Ve seen this often in the past and this have been reported here too)

Try to refresh page (sometimes it’s enough to fix that)
Now workaround:
Use the only when section or conditionnal that is empty to set Dropdown Type’s value, right click to copy expression (even if this is red), and paste it in type of your search. Clear the only when or conditionnal you used to put the expression.