I've never seen 'slots:' in a class without a visibility specification. It might not be the error, but it is worth a try. In your case, I guess that would be 'private slots:'.
Also, you might be interested in auto-connect slots. It's a Qt slot naming convention (on_<object>_<signal>). If you call your slot 'on_this_clicked()', you don't need to explicitly connect.
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