the error comes due to many issues adapter not configured properly , MSAD user not sync or new user etc.
Some of them which are ideal solution to fix are listed below.
Error: Object reference not set to an instance of an object. at line: 121" (Doc ID 1470967.1)
Automate error is usually related to script errors.
Do you have any event script for Validate step (BefValidate, AftValidate..)?