simplenews_send_source() results in PHP notices ("Trying to get property of non-object")

Issue Summary

I have a case of many notices per cron run.

The methods...
SimplenewsSourceNode->getLanguage() (multiple times)
SimplenewsSourceNode->getRecipient() and
SimplenewsSourceNode->setContext()
...all try to return properties of an invalid $source.

By investigating the code a little bit, I think all originate from simplenews_get_spool() which must be in my current setup be returning invalid sources.

My best guess is, there are subscriptions of deleted accounts. Cron likely tries to spool their newsletters. The mailing always fails, so they remain in the spooler. Just a guess!

Setting issue to major, because it accumulates and is capable on a site with many users to kill simplenews spooler (or even cron?) and flood watchdog.

A quick fix might be is to check the $source in simplenews_send_source() and - in case of an invalid object - assume success to proceed with the healthy spooler cleanup. This would not heal the original issue, but be helpful on affected production environments. I'd provide such a patch in case the issue doesn't get fixed contemporary.