You can’t use the Test-ExchangeSearch cmdlet for monitoring or testing in Microsoft Exchange Server 2016. Monitoring application by running the Test-ExchangeSearch cmdlet causes the Powershell application pool to crash. In addition, the following error message is logged:
Log Name: Application
Source: ASP.NET 4.0.30319.0
Event ID: 1325
Task Category: None
Description: An unhandled exception occurred and the process was terminated.
Application ID: DefaultDomain
Process ID: 7332
Exception: System.NullReferenceException Message: Object reference not set to an instance of an object.
To fix this issue, install the Cumulative Update 16 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Learn about the terminology that Microsoft uses to describe software updates.