Last week I came across an issue when attempting to create a new custom synchronisation rule in Azure AD Connect. When I tried to finish the wizard and add the rule, I received the error: “Object reference not set to an instance of an object”.
The workaround is to add a tag to the rule on the Description page, as shown below. The tag doesn’t need to be meaningful.
The issue appears to be specific to version 1.1.561.0 (July 2017) of AAD Connect. It wasn’t a problem in previous versions and it has been addressed in version 1.1.614.0 (September 2017).
For the latest of AAD Connect versions and version history, see https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-version-history