Update

src/main/java/com/github/kerbity/kerb/server/Server.java

@@ -149,8 +149,9 @@ public byte[] getHashedPassword(byte[] salt) {
         return this.connectionList;
     }
 
-    private void cleanConnectionList() {
+    public void cleanConnectionList() {
         this.connectionList.remove(null);
+        if (this.connectionList.contains(null)) this.cleanConnectionList();
     }
 
     /**

src/main/java/com/github/kerbity/kerb/server/ServerConnection.java

@@ -363,6 +363,7 @@ public void removeDuplicateNames() {
             if (!serverConnection.getName().equals(this.getName())) continue;
             serverConnection.disconnect();
         }
+        this.server.cleanConnectionList();
     }
 
     private boolean validate() {

src/main/java/com/github/kerbity/kerb/server/command/command/ListCommand.java

@@ -28,6 +28,8 @@
 import com.github.minemaniauk.developertools.console.Logger;
 import org.jetbrains.annotations.NotNull;
 
+import java.util.ArrayList;
+
 /**
  * Represents the list command.
  * Used to list the connected clients to the server.
@@ -47,6 +49,12 @@ public void execute(@NotNull Server server, @NotNull String command) {
         logger.log("&rServer Connections &e" + server.getConnectionList().size());
 
         for (ServerConnection connection : server.getConnectionList()) {
+
+            if (connection == null) {
+                logger.warn("Something went wrong! One of the connections returned null.");
+                continue;
+            }
+
             RegisteredClient client = connection.getRegisteredClient();
             logger.log("&7{identifier} &r{name} &7is_valid:&7{is_valid}"
                     .replace("{identifier}", client.getIdentifier())