Events: Trigger a warning if the default action is not callable

This adds a warning in the case that the default action is not null but also not callable which can happen when the supplied method is not public.

This warning is also triggered in the new test case from commit 16ca217de8ce72a9c736f35f1a62718437369d88 when commit a8795974051a91137b01ff88dbf5586a647b24ce is missing (and then it will also be triggered on normal request).

An alternative is replacing the ``is_callable``-check by ``is_null`` which might give better errors when the actual execution fails.