[JENKINS-72561] Fix sidepanel.jelly not being loaded when build/project is not available (plug-ins, folders, etc.)

[kinow]

Implements suggestion from Victor (https://issues.jenkins.io/browse/JENKINS-72561), to be more defensive on loading sidepanel.jelly.

Pending some manual tests to confirm everything is working fine after this change.

Testing done

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[kinow]

Tested manually, worked the same as before. Hoping this will fix the issue for users that found the sidepanel.jelly error.