Handle null JSON extension gracefully (#6814)

* Handle null extension JSON value gracefully

* Better handling

* Spotless

Author: jamesagnew

hapi-fhir-base/src/main/java/ca/uhn/fhir/parser/JsonParser.java

@@ -55,6 +55,7 @@
 import ca.uhn.fhir.rest.api.EncodingEnum;
 import ca.uhn.fhir.util.ElementUtil;
 import ca.uhn.fhir.util.FhirTerser;
+import jakarta.annotation.Nonnull;
 import org.apache.commons.lang3.StringUtils;
 import org.apache.commons.lang3.Validate;
 import org.apache.commons.text.WordUtils;
@@ -1474,17 +1475,19 @@ private void parseExtension(ParserState<?> theState, BaseJsonLikeArray theValues
 		int allUnderscoreNames = 0;
 		int handledUnderscoreNames = 0;
 
+		if (theValues == null) {
+			String parentElementName = getExtensionElementName(theIsModifier);
+			getErrorHandler()
+					.missingRequiredElement(new ParseLocation().setParentElementName(parentElementName), "url");
+			return;
+		}
+
 		for (int i = 0; i < theValues.size(); i++) {
 			BaseJsonLikeObject nextExtObj = BaseJsonLikeValue.asObject(theValues.get(i));
 			BaseJsonLikeValue jsonElement = nextExtObj.get("url");
 			String url;
 			if (null == jsonElement || !(jsonElement.isScalar())) {
-				String parentElementName;
-				if (theIsModifier) {
-					parentElementName = "modifierExtension";
-				} else {
-					parentElementName = "extension";
-				}
+				String parentElementName = getExtensionElementName(theIsModifier);
 				getErrorHandler()
 						.missingRequiredElement(new ParseLocation().setParentElementName(parentElementName), "url");
 				url = null;
@@ -1553,6 +1556,17 @@ private void parseExtension(ParserState<?> theState, BaseJsonLikeArray theValues
 		}
 	}
 
+	@Nonnull
+	private static String getExtensionElementName(boolean theIsModifier) {
+		String parentElementName;
+		if (theIsModifier) {
+			parentElementName = "modifierExtension";
+		} else {
+			parentElementName = "extension";
+		}
+		return parentElementName;
+	}
+
 	private void parseFhirComments(BaseJsonLikeValue theObject, ParserState<?> theState) {
 		if (isSupportsFhirComment()) {
 			if (theObject.isArray()) {

hapi-fhir-docs/src/main/resources/ca/uhn/hapi/fhir/changelog/8_2_0/6814-gracefully-handle-null-json-extension.yaml

@@ -0,0 +1,7 @@
+---
+type: add
+issue: 6814
+title: "When parsing a JSON resource, the following fragment caused the
+  parser to abort parsing with an unhelpful error message:
+  `\"extension\": null`. We will now correctly detect this error and pass
+    it to the parser error handler."