Ah dang it. I think the last two notices must be due to the option "close questions with a selected answer" which I didn't test in PHP 7.4. You can fix it by replacing the two lines in qa-include/pages/question-view.php with this:
line 434
if (isset($closepost['basetype']) && $closepost['basetype'] == 'Q') {
line 451
} elseif (isset($closepost['type']) && $closepost['type'] == 'NOTE') {
The first two are likely from a plugin, can you try disabling each plugin in turn and see if the notices go away? And let me know which plugin it occurred in.