Ensure periodic callback cleanup does not error on session warmup

[philippjfr]

Fixes #6838

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.51%. Comparing base (fa1c9fc) to head (b5a8cc4).
Report is 2 commits behind head on main.

Files	Patch %	Lines
panel/io/callbacks.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6886      +/-   ##
==========================================
- Coverage   81.52%   81.51%   -0.02%     
==========================================
  Files         319      319              
  Lines       46934    46935       +1     
==========================================
- Hits        38265    38259       -6     
- Misses       8669     8676       +7     

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