Fix VanishStatusChangeEvent NPE and document parameter inversion

Essentials/src/main/java/net/ess3/api/events/VanishStatusChangeEvent.java

@@ -8,6 +8,12 @@
  * <p>
  * For other cases where the player's vanish status changes, you should listen on PlayerJoinEvent and
  * check with {@link IUser#isVanished()}.
+ *
+ * <b>WARNING: The values of {@link #getAffected()} and {@link #getController()} are inverted due to
+ * a long-standing parameter swap. {@link #getAffected()} returns the command sender (null for console),
+ * and {@link #getController()} returns the player whose vanish status changed.</b>
+ *
+ * @see <a href="https://github.com/EssentialsX/Essentials/issues/2604">#2604</a>
  */
 public class VanishStatusChangeEvent extends StatusChangeEvent {
     private static final HandlerList handlers = new HandlerList();

EssentialsDiscord/src/main/java/net/essentialsx/discord/listeners/BukkitListener.java

@@ -126,14 +126,16 @@ public boolean isSilentJoinQuit(final IUser user, final String type) {
 
     @EventHandler(priority = EventPriority.MONITOR, ignoreCancelled = true)
     public void onVanishStatusChange(VanishStatusChangeEvent event) {
-        if (!jda.getSettings().isVanishFakeJoinLeave() || event.getAffected().isLeavingHidden()) {
+        // Note: getController() returns the vanished player due to a long-standing parameter swap in Commandvanish.
+        final IUser vanished = event.getController();
+        if (vanished == null || !jda.getSettings().isVanishFakeJoinLeave() || vanished.isLeavingHidden()) {
             return;
         }
         if (event.getValue()) {
-            sendJoinQuitMessage(event.getAffected().getBase(), ChatColor.YELLOW + event.getAffected().getName() + " left the game", MessageType.DefaultTypes.LEAVE);
+            sendJoinQuitMessage(vanished.getBase(), ChatColor.YELLOW + vanished.getName() + " left the game", MessageType.DefaultTypes.LEAVE);
             return;
         }
-        sendJoinQuitMessage(event.getAffected().getBase(), ChatColor.YELLOW + event.getAffected().getName() + " joined the game", MessageType.DefaultTypes.JOIN);
+        sendJoinQuitMessage(vanished.getBase(), ChatColor.YELLOW + vanished.getName() + " joined the game", MessageType.DefaultTypes.JOIN);
     }
 
     public void sendJoinQuitMessage(final Player player, final String message, MessageType type) {