No warning shown for undefined title callback function

Thanks for working on this. I have added a simple test to see what this change means. The issue is a bit old and now PHP is doing more strict checks in various functions. Something it was previously a warning is now a TypeError, see the tests on PHP 8.1: https://git.drupalcode.org/project/drupal/-/jobs/1366920

I do not think that changing the current state (no error, no information about the mistake) to the proposed state (information via error) is a good idea. I think that maybe a simple watchdog message in case of undefined title callback will be a more viable solution. Thanks!

Yes, new PHP versions throw an exception, instead of a PHP warning, but that happens also when the used page callback does not exist or it is not found. The MR I created made the behavior equal in the "title callback not existing or not found" and the "page callback not existing or not found", which I thought what this issue was trying to achieve.

I like better a logged message too, but I guess the same should be done also for page callbacks, which are invoked with call_user_func_array().

The exception that causes run-tests.sh to fail is the following one.

PDOException: SQLSTATE[42S02]: Base table or view not found: 1146 Table "jenkins_drupal_d7_299361.simpletest106828cache_block" doesn't exist

It is not an exception caused by the MR's change.