[BUG] Server fails to setup when using background callbacks without cancel argument

[tlauli]

Describe your context
Please provide us your environment, so we can easily reproduce the issue.

• replace the result of pip list | grep dash below

dash                                     2.12.0
dash-bootstrap-components                1.4.2
dash-bootstrap-templates                 1.0.8
dash-core-components                     2.0.0
dash-html-components                     2.0.0
dash-table                               5.0.0

• if frontend related, tell us your Browser, Version and OS

  • OS: [e.g. iOS]
  • Browser [e.g. chrome, safari]
  • Version [e.g. 22]

Describe the bug

Server fails to setup when using background callbacks without cancel argument.
This condition is not triggered, causing a KeyError here.

Expected behavior

Server setups properly. Adding None as default to the offending pop seemed to fix the issue in my quick test.