Clicking timestamps doesn't work until org-agenda is called manually

[Michael Maurer]

Subject line basically states the problem; when I start Emacs and try
to click a timestamp such as <2023-01-16 Mo.>, I get the message
"Symbol's value as variable is void: org-agenda-buffer-name". However,
once org-agenda is called, repeating the action results in the
org-agenda view.