Properly clear channels and channel users on disconnect

Author: Sheikah45

src/main/java/com/faforever/client/chat/ChatChannel.java

@@ -17,6 +17,7 @@
 import lombok.Getter;
 import lombok.ToString;
 
+import java.util.List;
 import java.util.Optional;
 import java.util.function.Consumer;
 
@@ -117,7 +118,7 @@ public ChatChannelUser createUserIfNecessary(String username, Consumer<ChatChann
   }
 
   public void clearUsers() {
-    usernameToChatUser.clear();
+    List.copyOf(usernameToChatUser.keySet()).forEach(this::removeUser);
   }
 
   public ObservableList<ChatChannelUser> getTypingUsers() {

src/main/java/com/faforever/client/chat/KittehChatService.java

@@ -537,7 +537,7 @@ private void onMessage(String message) {
   private void onDisconnect(ClientConnectionEndedEvent event) {
     client.getEventManager().unregisterEventListener(this);
     channels.values().forEach(ChatChannel::clearUsers);
-    channels.clear();
+    List.copyOf(channels.keySet()).forEach(this::removeChannel);
     connectionState.set(ConnectionState.DISCONNECTED);
     client.shutdown();
   }