Don't error out if weak handle isn't present

src/states.rs

@@ -356,12 +356,16 @@ impl States {
         let logging = self.log_state_change;
 
         Ok(Box::new(move || {
+            if let Some(current) = weak_current_state.upgrade() {
                 Self::_set_state(
                     weak_state.as_ref().map(|w| w.upgrade()).flatten(),
-                &mut *weak_current_state.upgrade().unwrap().lock().unwrap(),
+                    &mut *current.lock().unwrap(),
                     gui_handler.as_ref().map(|h| h.upgrade()).flatten(),
                     logging,
-            )
+                )?;
+            }
+
+            Ok(())
         }))
     }
 }