Mirroring: "Object reference not set to an instance of an object" at validating mirroring settings

Description

Steps to reproduce:
1. Upgrade a 7.5 CMS site to 10.1.0.0.
2. Configure mirroring for mirror from Page1 to Page2 -> mirroring settings are valid and its function runs properly.
3. Upgrade to CMS Core 10.2.x.
4. Open Config > Mirroring > check system for the mirroring settings

Expected: Validation is OK
Actual: Error message appears in Messages column as "Object reference not set to an instance of an object." (this error message is not clear/informative for users while no log is written in log file)
Observed: After upgrading to Core 10.3.0, this bug is not seen but QA can't make sure that it may occur again after fixing the bug CMS-5997