Magento2 – Incorrect Theme Identification Key

Any time I run bin/magento setup:upgrade I get Incorrect theme identification key

I understand the source is vendor/magento/framework/View/Design/Theme/FlyweightFactory.php
so I had added echos to try and figure something out

I get

First pass 1, second pass NULL

Now, would be nice to understand where 1 and NULL come from

mysql> select theme_id, theme_path, area from theme;
+----------+-----------------+-----------+
| theme_id | theme_path      | area      |
+----------+-----------------+-----------+
|        1 | Magento/blank   | frontend  |
|        2 | Magento/backend | adminhtml |
|        3 | Magento/luma    | frontend  |
+----------+-----------------+-----------+

mysql> select * from core_config_data where path like '%theme%';
+-----------+----------+----------+-----------------------+-------+
| config_id | scope    | scope_id | path                  | value |
+-----------+----------+----------+-----------------------+-------+
|        34 | stores   |        1 | design/theme/theme_id | 3     |
|        58 | websites |        1 | design/theme/theme_id | 3     |
|        82 | default  |        0 | design/theme/theme_id | 3     |
+-----------+----------+----------+-----------------------+-------+

Only theme set is 3 (Luma), so I don't get how 1 gets passed.
I don't know where the "crontab" area theme is set. Is there anywhere I can set that value so I won't get the error?

Any help is appreciated, happy to provide more info.