validate-code on remote-terminology service returns NullPointerException

[volodymyr-korzh]

With remote-terminology service enabled, $validate-code operation with unknown code system or unknown Value Set returns 500 Server Error response with NullPointerException in body. With remote-terminology service disabled - response is correct.

Steps To Reproduce:

1. Enable Remote terminology Service
2. Send a request with invalid Code System to fhir endpoint:
   POST: {fhir-endpoint}/CodeSystem/$validate-code

{
	"resourceType": "Parameters",
	"parameter": [
		{
			"name": "url",
			"valueUri": "http://terminology.hl7.org/CodeSystem/unknown"
		},
		{
			"name": "coding",
			"valueCoding": {
				"code": "P",
				"system": "http://terminology.hl7.org/CodeSystem/unknown"
			}
		}
	]
}

3. Send a request with invalid Value Set to fhir endpoint:
   POST: {fhir-endpoint}/ValueSet/$validate-code

{
	"resourceType": "Parameters",
	"parameter": [
		{
			"name": "url",
			"valueUri": "http://hl7.org/fhir/ValueSet/uknown-value-set"
		},
		{
			"name": "coding",
			"valueCoding": {
				"code": "P",
				"system": "http://terminology.hl7.org/CodeSystem/v2-0247"
			}
		}
	]
}

Actual result
500 Server Error

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

Expected behavior
Response should be the same, as without remote terminology service enabled:
200 OK

{
	"resourceType": "Parameters",
	"parameter": [
		{
			"name": "result",
			"valueBoolean": false
		},
		{
			"name": "message",
			"valueString": "Validator is unable to provide validation for P#http://terminology.hl7.org/CodeSystem/v2-0247 - Unknown or unusable ValueSet[http://hl7.org/fhir/ValueSet/uknown-value-set]"
		}
	]
}