fix blocking issue when cell contains ipywidget

Author: huajie.liu

nbclient/client.py

@@ -768,10 +768,12 @@ async def _async_poll_for_reply(
     ) -> dict[str, t.Any]:
         msg: dict[str, t.Any]
         assert self.kc is not None
-        new_timeout: float | None = None
         if timeout is not None:
             deadline = monotonic() + timeout
             new_timeout = float(timeout)
+        else:
+            deadline = monotonic() + 5
+            new_timeout = float(5)
         error_on_timeout_execute_reply = None
         while True:
             try:
@@ -800,7 +802,10 @@ async def _async_poll_for_reply(
                         new_timeout = max(0, deadline - monotonic())
             except Empty:
                 # received no message, check if kernel is still alive
-                assert timeout is not None
+                if timeout is None:
+                    deadline = monotonic() + 5
+                    new_timeout = float(5)
+                    continue
                 task_poll_kernel_alive.cancel()
                 await self._async_check_alive()
                 error_on_timeout_execute_reply = await self._async_handle_timeout(timeout, cell)