serverSocketChannel.accept() returns null

Question

I am trying to write a simple client server program in Java NIO.

Here is the code:

package org.lb;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Set;

public class Test {
  public static void main(String[] args) throws IOException {
    ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
    serverSocketChannel.bind(new InetSocketAddress(8080));
    serverSocketChannel.configureBlocking(false);

    Selector selector = Selector.open();
    serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

    while (true) {
      selector.select();
      Set<SelectionKey> selectionKeySet = selector.selectedKeys();
      for (SelectionKey selectionKey : selectionKeySet) {
        if (selectionKey.isAcceptable()) {
          SocketChannel socketChannel = serverSocketChannel.accept();
          if (socketChannel == null) {
            System.out.println("No connection...");
          } else {
            socketChannel.configureBlocking(false);
            socketChannel.register(selector, SelectionKey.OP_READ);
            System.out.println("Connection done!");
          }
        }

        if (selectionKey.isReadable()) {
          System.out.println("isReadable");
          SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
          socketChannel.close();
        }
      }
    }
  }
}

You can see that I call serverSocketChannel.accept() only when selectionKey.isAcceptable() returns true. So, I assume that serverSocketChannel.accept() should always return a valid SocketChannel instance, but sometimes it's returning null as well.

I am trying to send request using Postman, but the same case is with browser requests too.

Output with Postman single request

Connection done!
No connection...
isReadable
Connection done!
No connection...
isReadable

I expected that Connection done! is printed once, but it's printing twice. Also, why No connection... is printed?

Output with curl single request

Connection done!
isReadable
No connection...