Fix crash on exit again

src/widgets/wm.rs

@@ -118,8 +118,13 @@ impl Widget for Bspwm {
     fn handle_event(&mut self, event: &Message) -> Update {
         match event {
             &Message::BspwmEvent(ref line) => {
-                self.parse_bspwm(line);
-                Update::Relayout
+                if !line.is_empty() {
+                    self.parse_bspwm(line);
+                    Update::Relayout
+                }
+                else {
+                    Update::Nothing
+                }
             },
             &Message::MousePress(x) => {
                 for desktop in self.desktops.iter() {