in grpc-web/callout/src/main/java/com/google/apigee/callouts/util/VarResolver.java [136:142]
public String getRequiredProp(String propName, String message) {
String result = getProp(propName, String.class, null);
if (result == null || result.isEmpty()) {
throw new RuntimeException(propName + " is required." + message);
}
return result;
}