User Warning: Form was submitted but there are no associated handlers

Forms since version 3.1.4 check if they have any handler, i.e. there is a method callback for onSuccess, onSubmit or onClick on the button:

$form->onSuccess[] = [$this, 'formSucceeded'];

If missing, it throws a warning Form was submitted but there are no associated handlers.

The reason is that it happens that a programmer, especially a beginner, forgets to associate a handler and then has a complicated search for why submitting the form doesn't work. This should make it easier for him to develop.