[BUG][OpenSearch Plugins] script_exception Runtime Error When Previewing Trigger Condition in Per Cluster Metrics Monitor

[getsolaris]

Describe the bug
When creating a Per Cluster Metrics Monitor in the OpenSearch Plugins tab, selecting "Nodes stats" as the request type and clicking on "Preview condition response" after setting the trigger results in a script_exception runtime error.

To Reproduce
Steps to reproduce the behavior:

1. Go to OpenSearch Dashboards.
2. Navigate to the "Plugins" tab and click on "Create Monitor."
3. Select "Per Cluster Metrics Monitor" as the monitor type.
4. For the query request type, choose "Nodes stats."
5. Click on "Preview query" to check the query response.
6. Set a trigger and leave the "Trigger condition" as default.
7. Click on "Preview condition response."
8. See the script_exception runtime error.

Expected behavior
The trigger condition preview should return a valid response without throwing a script_exception runtime error.

OpenSearch Version
OpenSearch 2.15 (OpenSearch_2_15_R20240904-P2)

Dashboards Version
v 2.15.0

Plugins
Please list all plugins currently enabled.

Screenshots

• Trigger condition

ctx.results[0].nodes.NODE_ID.jvm.mem.heap_used_percent > 60

• Trigger condition response

{
  "type" : "script_exception",
  "reason" : "runtime error",
  "script_stack" : [
    "ctx.results[0].nodes.NODE_ID.jvm.mem.heap_used_percent > 60",
    "                            ^---- HERE"
  ],
  "script" : "ctx.results[0].nodes.NODE_ID.jvm.mem.heap_used_percent > 60",
  "lang" : "painless",
  "position" : {
    "offset" : 28,
    "start" : 0,
    "end" : 59
  },
  "caused_by" : {
    "type" : "null_pointer_exception",
    "reason" : "Cannot invoke \"Object.getClass()\" because \"callArgs[0]\" is null"
  }
}

Host/Environment (please complete the following information):

• OS: Mac OS
• Browser and version: Version 129.0.6668.100 (Official Build) (arm64)

Additional context
This error only occurs when trying to preview the trigger condition. Query preview works as expected, and the error appears during the preview of the trigger condition response.

[zhyuanqi]

This belongs to alerting plugin. Reroute it to alerting.

[zhyuanqi]

@opensearch-project/plugins could you please transfer to the right repo?

[getsolaris]

@zhyuanqi thank you

[getsolaris]

opensearch-project/notifications#978