I have had this problem before and I either didn't ask about it here, or I found it by searching then and forgot how to phrase the question to find it again.
I'm using the eSearch in the WAB.
I go to build a search and I enter the search URL, the title autopopulates, I click the options I want (Spatial Search Layer, Add Result as Operational Layer and Show in Attribute Table Widget) and then craft my expression. I set a SINGLE expression... simple... (or complex, it doesn't matter) and when I click [OK] and I am brought back to the "Update Search Layer" window in the Widget the search expression has a nameless (and now empty) search expression in it. I can hover over it and there is an expression with the arrows to move it up or down the list and an X to delete it, but no name (despite naming it) AND when I click the edit option I am taken to a now empty Expression page.
It's happened a few times. I've rebuilt the web map from scratch thinking it was a glitch in the WAB... but nothing.
I can't craft a Search Layer now...
Not sure what happened...