code: public void nextNull() throws IOException {
      peek();
      if (token != JsonToken.NULL) {
        throw new IllegalStateException("Expected null but was " + token
            + " at line " + getLineNumber() + " column " + getColumnNumber());
      }
  
      advance();
    }
docs: Consumes the next token from the JSON stream and asserts that it is a
  literal null.
  
  @throws IllegalStateException if the next token is not null or if this
  reader is closed.