Hello
This is Beda, and Nigel has asked me to help out here.
I will try this what you suggest (use Types 1.7 and then upgrade).
I tried to log in to the Test site but seems you removed the access already.
So I will need to quickly deploy the Duplicator (if possible) and study your Videos you provided.
I'll get back at you as soon I have replicated or not, and if yes, solved.
Types 1.7. and ACF using a relationship Field with all default settings on a Custom Post type, creates this:
"Notice: Trying to get property of non-object in plugins/advanced-custom-fields-pro/api/api-helpers.php on line 1146"
Anyway, I then updated to last Types and everything works just fine.
I can see the relationships and then Choose the Posts or other items in the Select2 Instance of ACF.
Please can you provide exact steps how I can replicate this problem?
All seems to work fine on my end.
You might want to try to re-save all involved elements once; perhaps something has been corrupted during the upgrade