QueueSink.get() returns None

If a `sink: QueueSink` is used to collect and sequentially process all incoming dataframes via `sink.get()` function, only None is returned. When using `sink.queue.get()` the dataframes are returned. This is probably, because `sink.get()` does not acquire the queues lock = it is not thread-safe. We should either make it thread-safe, or remove the get() function.