When clicking on the "Machine Catalog" node in studio, custom administrators with a limited scope assignment defined receive the error "Object reference not set to an instance of an object".

Add the scope(s) assigned to the custom administrator to the Hosting Connection(s) that correspond to the Machine Catalog(s) that the administrator is attempting to access.

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Problem Cause

If a custom administrator with limited scope permissions requires read or edit access to Machine Catalogs containing machines provisioned via MCS or PVS, the scope assignment must also be applied to the Hosting Connection(s) that corresponds to the Machine Catalog(s) the administrator must access. Edit the scope of the Hosting Connection by selecting the Hosting Connection, select Edit Connection, select Scopes, and chose the scope(s) that are assigned to the administrator.

When delegated administrators are assigned custom scopes, the same custom scopes need to also be assigned to the Catalogs and corresponding hypervisor.

https://docs.citrix.com/en-us/citrix-virtual-apps-desktops-service/manage-deployment/delegated-administration.html
 

https://docs.citrix.com/en-us/citrix-virtual-apps-desktops-service/setup-for-citrix-service-providers.html