NVP-vProxy: NMC Virtual Machine Group Preview Fails "Cannot invoke "COM.legato.gst.GtValueList.Iterator()" because "children" is null"

Dynamic Rules are configured for NetWorker VMware Protection (NVP): NetWorker: vProxy: How to enable a VMware group with Dynamic Association and Rules
The NetWorker Management Console (NMC) group properties window includes a "Preview All Virtual Machines" option.
Using this option for one or more groups produces the following error in the NMC:
 

Error: Cannot invoke "COM.legato.gst.GtValueList.Iterator()" because "children" is null

This is observed on large protection groups, spanning hundreds (500+) to thousands of Virtual Machines (VM) in a single group.
Only the NMC preview function is impacted:

• Backup operations on the group succeed.
• The NetWorker Web User Interface (NWUI) "Preview" option lists the Virtual Machines.

This issue was raised to NetWorker engineering.

This is an architectural limitation with the legacy NetWorker Management Console (NMC) which has been improved in the NetWorker Web User Interface (NWUI). These components use two different data collectors:

• NWUI: Uses REST-based communication: previewWorkItemSelection → VCenterDataAdapter.getMergedWorkitems → VimWrapper.getMergedWorkItemsWithJsonRequest → vproxyInfrastructure.previewVMs
  This workflow returns JSON over HTTP and can handle large payloads efficiently, which is well-suited for large-scale VMware environments.
• NMC: Uses the legacy GSTD protocol rather than REST. While it leverages the same underlying VM data, it is processed through the legacy GST Java object model (GtValue, GtValueList), or An intermediate "job as JSON" representation. The observed error occurs within this handling path (COM.legato.gst.GtValueList.Iterator), indicating a limitation in how large datasets are processed.

NetWorker is working as designed.

Use the NetWorker Web User Interface (NWUI) "preview" option in the group settings:

1. Go to https://<NetWorker_Server_Address>:9090/nwui
2. Log in with your NetWorker account. (For example, default Administrator account)
3. Go to Protection → Groups.
4. Select a VMware protection group, then click Edit,
5. Click the VMware Resources tab.
6. Click Preview.

The NetWorker VMware Integration Guide states the following:

• Ensure that you do not exceed more than 10 properties or conditions inside a rule definition. There is no limit on the number of rules that can be created in a given NetWorker server, the mentioned limit here applies only to the number of properties or conditions in a single rule definition. Ensure that you do not combine multiple vCenters into the same rule, that is, using one single rule for all the vCenters in your environment is not recommended. We recommend you to have a separate rule created for each vCenter in your environment.
• Total number of virtual machines in a single NetWorker policy:
  • Maximum: 2000
  • Recommended: 1000
  • You can run multiple vProxy policies concurrently if the total number of concurrent backup streams does not exceed the vCenter limits (100 concurrent sessions). For a single vCenter, stagger the schedules for policies to ensure that all the backups for a policy are complete before the backups of the next policy begin.

NetWorker documentation is available through: Support for NetWorker | Manuals & Documents (You must sign i with your Dell support account).