String getFirstValue(String name) {
            final String value = getFirstValueOrNull(name);
            if (value == null) {
                throw new InvalidSamlRequestException("failed to get the value of a parameter: " + name);
            }
            return value;
        }
Returns the first value of the parameter with the specified {@code name}.

@throws InvalidSamlRequestException if a parameter with the specified {@code name} does not exist
saml/src/main/java/com/linecorp/armeria/server/saml/SamlService.java