code: public static Validator<Object> notNull(@NonNull final Context context) {
          return new NotNullValidator(context, R.string.default_error_message);
      }
docs: Creates and returns a validator, which allows to ensure, that values are not null.
  
  @param context
  The context, which should be used to retrieve the error message, as an instance of
  the class {@link Context}. The context may not be null
  @return The validator, which has been created, as an instance of the type {@link Validator}