code: protected static void failIfNotNull(@Nullable String customErrorMessage, @Nullable Description description,
                                        @Nullable Object o) {
      if (o == null) {
        return;
      }
      failWithMessage(customErrorMessage);
      fail(description, array(inBrackets(o), " should be null"));
    }
docs: Throws an {@code AssertionError} if the given object is not {@code null}.
  
  @param customErrorMessage any custom error message. This message will replace the default one only if it (the
  custom message) is not {@code null}.
  @param description        the description of the given object.
  @param o                  the given object.
  @throws AssertionError if the given object is not {@code null}.
  @since 1.2