Fix possibly not defined tracker

[davidbrochart]

See this error:

Traceback (most recent call last):
  File "/Users/runner/work/ipykernel/ipykernel/ipykernel/iostream.py", line 175, in _handle_event
    raise e
  File "/Users/runner/work/ipykernel/ipykernel/ipykernel/iostream.py", line 171, in _handle_event
    event_f()
  File "/Users/runner/work/ipykernel/ipykernel/ipykernel/iostream.py", line 638, in _flush
    self.session.send(
  File "/Users/runner/Library/Application Support/hatch/env/virtual/ipykernel/8WkbHHn4/cov.qt6/lib/python3.11/site-packages/jupyter_client/session.py", line 870, in send
    msg["tracker"] = tracker
                     ^^^^^^^
UnboundLocalError: cannot access local variable 'tracker' where it is not associated with a value

[davidbrochart]

Failures are unrelated.

[blink1073]

Thanks!