code: public static <T> T notNull(T argument, String argumentName) {
          if (argument == null) {
              throw new IllegalArgumentException("Argument " + argumentName + " must not be null.");
          }
          return argument;
      }
docs: Checks if the given argument is null and throws an exception with a
  message containing the argument name if that it true.
  
  @param argument     the argument to check for null
  @param argumentName the name of the argument to check.
  This is used in the exception message.
  @param <T>          the type of the argument
  @return the argument itself
  @throws IllegalArgumentException in case argument is null