Add null check to channel close

common/src/main/java/us/myles/ViaVersion/api/platform/ViaConnectionManager.java

@@ -20,8 +20,10 @@ public class ViaConnectionManager {
         connections.add(connection);
         clients.put(id, connection);
 
+        if (connection.getChannel() != null) {
             connection.getChannel().closeFuture().addListener((ChannelFutureListener) future -> onDisconnect(connection));
         }
+    }
 
     public void onDisconnect(UserConnection connection) {
         Objects.requireNonNull(connection, "connection is null!");