Fix NPE when request is empty / host is not given

When the request is not valid, the response will be null, leading to a NPE.
After the try block, the null response will be caught and an error will be
logged which is better than throwing a NPE.
Lock debugging: print stack trace for readers
Lock debugging: Fix ConcurrentModificationException

unsynchronized access to the readers_ property causes a said exception. Since
the super class synchronizes on this, let's do the same here. It should be
sufficient to synchronize only when getting the iterator and looping through it,
but that would change more lines (indentation), therefore I chose to
synchronize the whole method.
WIP: debugging lock issue