Minecraft server starts but plugin does not work what to do?

I created a local server last night and wanted to start programming with the Bukkit, which was a lot of fun for me.

I then uploaded the plugin to the plugins folder and quickly noticed that something was wrong because I programmed something as a join message.

Now I have the following problem in my ServerLog:

01:53:35 ERROR]: Exception handling console input
java.io.IOException: The handle is invalid
at java.io.FileInputStream.readBytes (Native Method) ~ [?:?]
at java.io.FileInputStream.read (FileInputStream.java:273) ~ [?:?]
at java.io.BufferedInputStream.read1 (BufferedInputStream.java:290) ~ [?:?]
at java.io.BufferedInputStream.read (BufferedInputStream.java:351) ~ [?:?]
at sun.nio.cs.StreamDecoder.readBytes (StreamDecoder.java:284) ~ [?:?]
at sun.nio.cs.StreamDecoder.implRead (StreamDecoder.java:326) ~ [?:?]
at sun.nio.cs.StreamDecoder.read (StreamDecoder.java:178) ~ [?:?]
at java.io.InputStreamReader.read (InputStreamReader.java:185) ~ [?:?]
at java.io.BufferedReader.fill (BufferedReader.java:161) ~ [?:?]
at java.io.BufferedReader.readLine (BufferedReader.java:326) ~ [?:?]
at java.io.BufferedReader.readLine (BufferedReader.java:392) ~ [?:?]
at wd $ 2.run (SourceFile: 110) [server.jar:?]
[01:53:35 INFO]: Starting Minecraft server on *: 25565

I don't quite understand what the problem is. I have already logically looked up the Internet but like no solution found.

Is this really a problem with the server or with the code?

What I also noticed is that if I enter absolutely nothing in the server / plugin, nothing is displayed, not even any message that there's this command.

Handle is invalid

And that means?

Have I ever said things where you need urgent white plugins?

I say as an example.

You program a scoreboard and just use a plugin with the pre-made coin system.

Then you would have to put your programmed and the coin system plugin in so that it works