src/main/java/org/fest/assertions/Fail.java
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"));
  }
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"));
  }
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