Comment 2 for bug 1161337

The problem seems to be that a search triggered by queue_search_changed sends the whitespace only string as search term but normally Unity doesn't send whitespace only searches to scopes.

To make matters worse, app-lens doesn't consider whitespace only search as empty (in Utils.is_search_empty) and an invalid query string is generated (and the search fails):
WARNING **: Error parsing query 'type:Application AND ((category:Accessibility AND NOT category:Settings)) AND ': Syntax: <expression> AND <expression>