fix #2937 unhandled error in dart_fn_handle_output when sending message

Author: khangah

frb_rust/src/dart_fn/handler.rs

@@ -58,7 +58,11 @@ impl DartFnHandler {
         let _ = panic::catch_unwind(move || {
             let catch_unwind_result = panic::catch_unwind(move || {
                 if let Some(completer) = (self.completers.lock().unwrap()).remove(&call_id) {
-                    completer.send(message).unwrap();
+                    if let Err(err) = completer.send(message) {
+                        log_warn_or_println(&format!(
+                            "Error in dart_fn_handle_output when sending message for call_id {call_id}: {err:?}"
+                        ));
+                    }
                 }
             });
             if let Err(err) = catch_unwind_result {