Fix kernel lookup

[davidbrochart]

This should fix the errors in #473 (https://github.com/jupyter-server/jupyter_server/pull/473/checks?check_run_id=2323090792#step:14:342), but this is not addressing the real root cause.

[codecov-io]

Codecov Report

Merging #475 (9fa0bbf) into master (02cc7d7) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #475      +/-   ##
==========================================
- Coverage   77.81%   77.71%   -0.10%     
==========================================
  Files         106      106              
  Lines        9136     9137       +1     
  Branches      979      979              
==========================================
- Hits         7109     7101       -8     
- Misses       1678     1687       +9     
  Partials      349      349              

Impacted Files	Coverage Δ
jupyter_server/services/kernels/kernelmanager.py	77.60% <100.00%> (-3.02%)	⬇️
jupyter_server/services/kernels/handlers.py	58.24% <0.00%> (-0.27%)	⬇️

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[davidbrochart]

Actually, it looks like the kernel was shut down (and thus removed from the connections) before we ask for the status in the front-end, so this change might be right.
Maybe this is because the new jupyter_client does things in a different order now that it's async?

[kevin-bates]

Thanks David. This change looks good.

Yes, with async we now get to deal with race conditions more frequently particularly between front-end and periodic callbacks (which triggered this one). (Ref: #472)