DateTimeZone::__construct(): Unknown or bad timezone () in DateTimeZone->__construct()

There is no circumstance in which a timezone may be empty or invalid.

Its likely malformed data came in from programmatic code, a bad widget, or a migration.

Please fix the source data in your field data tables.

Just an FYI, we ran into this and it wasn't the data being wrong, it was a views aggregated field that needed timezone added to the "Group Columns" or it didn't have access to the value.
Hope that helps anybody struggling with this one and pulling their hair out as we were.

We were having this issue when editing an event node with a recurring date field, then clicking Preview, then Return to content editing. One additional error popped up, however, when using the patch from #3 so I'm posting a slight modification to that patch that seems to be more stable for us and resolves the errors that we've seen.

Thanks for sharing, the patch seems to suggest that maybe we are stepping on our own feet here. At that point in the code was the time zone already sent.?