code: public static <T /*>>> extends @NonNull Object*/> T checkNotNull(
        T arg, @javax.annotation.Nullable Object errorMessage) {
      if (arg == null) {
        throw new NullPointerException(String.valueOf(errorMessage));
      }
      return arg;
    }
docs: Throws a {@link NullPointerException} if the argument is null. This method is similar to {@code
  Preconditions.checkNotNull(Object, Object)} from Guava.
  
  @param arg the argument to check for null.
  @param errorMessage the message to use for the exception. Will be converted to a string using
  {@link String#valueOf(Object)}.
  @return the argument, if it passes the null check.