api/src/main/java/com/velocitypowered/api/proxy/Player.java

@@ -197,7 +197,7 @@ public interface Player extends
    *
    * @param reason component with the reason
    */
-  void disconnect(Component reason);
+  void disconnect(@NotNull Component reason);
 
   /**
    * Sends chat input onto the players current server as if they typed it into the client chat box.

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -111,6 +111,7 @@ import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Locale;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
 import java.util.UUID;
@@ -645,7 +646,8 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player,
   }
 
   @Override
-  public void disconnect(Component reason) {
+  public void disconnect(@NotNull Component reason) {
+    Objects.requireNonNull(reason, "reason");
     if (connection.eventLoop().inEventLoop()) {
       disconnect0(reason, false);
     } else {