Fix "No Session" in plugin messaging

[Smart123s]

Summary of your change

After Bungee recieved a plugin message from Bukkit, that it has completed login/register for a Floodgate player, bungee would throw a NullPointerException:

java.util.concurrent.CompletionException: java.lang.NullPointerException: Cannot invoke "com.github.games647.fastlogin.bungee.BungeeLoginSession.getProfile()" because "loginSession" is null
at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315) ~[?:?]
at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320) ~[?:?]
at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1807) ~[?:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[?:?]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[?:?]
at java.lang.Thread.run(Thread.java:833) [?:?]
Caused by: java.lang.NullPointerException: Cannot invoke "com.github.games647.fastlogin.bungee.BungeeLoginSession.getProfile()" because "loginSession" is null
at com.github.games647.fastlogin.bungee.listener.PluginMessageListener.onSuccessMessage(PluginMessageListener.java:131) ~[?:?]
at com.github.games647.fastlogin.bungee.listener.PluginMessageListener.readMessage(PluginMessageListener.java:92) ~[?:?]
at com.github.games647.fastlogin.bungee.listener.PluginMessageListener.lambda$onPluginMessage$0(PluginMessageListener.java:84) ~[?:?]
at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1804) ~[?:?]
... 3 more

Related issue

Strangely, no one has reported this issue. It exist in yesterday's build , idk about later versions, I haven't tested it.