FHIR Patch replace operation fails when replacing elements in a multi cardinality elements in FHIR resources when using FHIR patch (application/fhir) syntax

[mrdnctrk]

Describe the bug
FHIR Patch with replace operation that tries to replace an child element of a high cardinality field fails.

To Reproduce
Steps to reproduce the behavior:

1. Create a Patient Resource that contains a high cardinality element such as identifier

{
    "resourceType": "Patient",
    "identifier": [
    { 
       "system": "sys1",
       "value": "val1"
    },
    { 
       "system": "sys2",
       "value": "val2"
    }
   ]
}

2. Try to Patch the Patient by replacing the second identifier:

PATCH /Patient/{patient-id}
-Content-Type: application/fhir

{
    "resourceType": "Parameters",
    "parameter": [
        {
            "name": "operation",
            "part": [
                {
                    "name": "type",
                    "valueCode": "replace"
                },
                {
                    "name": "path",
                    "valueString": "Patient.identifier[1]"
                },
                {
                    "name": "value",
                    "valueIdentifier": { 
                        "system": "sys3", 
                        "value": "value3"
                    }
                }
            ]
        }
    ]
}

3. See 500 Response with the response:

{
    "resourceType": "OperationOutcome",
    "issue": [
        {
            "severity": "error",
            "code": "processing",
            "diagnostics": "HAPI-0389: Failed to call access method: java.lang.NullPointerException: Cannot invoke \"ca.uhn.fhir.context.BaseRuntimeChildDefinition.getValidChildNames()\" because \"childDef\" is null"
        }
    ]
} 

Expected behavior
The patch operation should succeed.

The same problem exists if path contain a where filter instead of index:

                {
                    "name": "path",
                    "valueString": "Patient.identifier.where(system='sys2')"
                }

Note: A similar operation succeeds when using json patch syntax.