For those who have encountered this error message, the cause can be an error in your hook code:
I spent 2 days going crazy until blueprint debugging tip https://forum.espocrm.com/forum/deve...gging-php-code helped me trace the origin which in my case was that I had changed the structure of a custom entity but had forgotten to adjust a hook for the same entity that I had written prior to the change.
Hopefully this will help someone else having this issue.
Espo.ERROR: Uncaught Exception TypeError: "Argument 1 passed to Espo\ORM\DB\Mapper::select() must implement interface Espo\ORM\IEntity, null given, called in C:\Bitnami\WAMP\apps\espocrm\htdocs\application\Es po\ORM\Repositories\RDB.php on line 208" at C:\Bitnami\WAMP\apps\espocrm\htdocs\application\Es po\ORM\DB\Mapper.php line 114
Hopefully this will help someone else having this issue.