Object reference not set to an instance of an object

Questions
1

Hi,
I’ve upgraded from Umbraco 7.3 to 7.12.4.

Everything worked perfectly, BUT:
from some reason the site itself stopped working. I’m getting
‘Object reference not set to an instance of an object.’ on

string testifierName = page.GetPropertyValue<string>("testifierName");

and basically on every line in my code that has the “GetPropertyValue”.
When debugging, I can see that instead of returning the value, the “GetPropertyValue” returns the type:

System.Collections.Generic.List`1[Umbraco.Core.Models.IPublishedContent]

I’m sure it’s web.config related, but I cannot seem to find the culprit.

Please help,
Thanks,
Rotem

This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/94981-object-reference-not-set-to-an-instance-of-an-object