FHIRPath nested extention issue

[tstrass]

If you attempt to evaluate a FHIRPath that has an extension somewhere other than the root object, the following error is thrown:

Extension function not applicable to NilClass:

To reproduce:

With this Patient

{
  "telecom": [
    {
      "system": "phone",
      "value": "(333) 333-3333",
      "extension": [
          {
            "url": "http://example.com/phone-type",
            "valueString": "Smartphone"
          }
      ]
    }
  ],
  "resourceType": "Patient"
}

If I call:

FHIRPath.evaluate("Patient.telecom[0].extension('http://example.com/phone-type')", patient_json)

I get the following output:

D, [2021-03-22T18:44:27.444782 #97061] DEBUG -- : TOKENS: ["Patient", "telecom[0]", "extension", "(", "'phone-type'", ")"]
D, [2021-03-22T18:44:27.445396 #97061] DEBUG -- : TREE: ["Patient", "telecom[0]", :extension, ["'phone-type'"]]
D, [2021-03-22T18:44:27.445681 #97061] DEBUG -- : DATA: ["Patient", "telecom[0]", :extension, ["'phone-type'"]]
D, [2021-03-22T18:44:27.456356 #97061] DEBUG -- : V===> [{"telecom"=>[{"system"=>"phone", "value"=>"(333) 333-3333", "rank"=>1, "extension"=>[{"url"=>"http://example.com/phone-type", "valueString"=>"Smartphone"}]}], "resourceType"=>"Patient"}, "telecom[0]", :extension, ["'phone-type'"]]
D, [2021-03-22T18:44:27.456710 #97061] DEBUG -- : V===> [nil, {"system"=>"phone", "value"=>"(333) 333-3333", "rank"=>1, "extension"=>[{"url"=>"http://example.com/phone-type", "valueString"=>"Smartphone"}]}, :extension, ["'phone-type'"]]
D, [2021-03-22T18:44:27.456932 #97061] DEBUG -- : Evaling Extension Block....
*** RuntimeError Exception: Extension function not applicable to NilClass:

As another example, I have also seen an error thrown when attempting to eval a FHIRPath to a US Core ethnicity code, since it is an extension nested within an extension.