NPE when player reconnects the proxy (proxy side error)

[Zulfen]

Description

Disconnecting then reconnecting to the proxy causes Skungee to seemingly lose the reference to the player's current server (?). This disconnects the backend from the proxy which also causes the backend servers to crash, like in my issue in #19, which this is probably closely related to.

Screenshots or Errors

>.... [17:50:16 ERROR]: Exception in thread "pool-5-thread-1" java.lang.NullPointerException
>.... [17:50:16 ERROR]: at com.skungee.bungeecord.BungeeSkungee.getCurrentServer(BungeeSkungee.java:170)
>.... [17:50:16 ERROR]: at com.skungee.bungeecord.BungeeSkungee.getPlayer(BungeeSkungee.java:157)
>.... [17:50:16 ERROR]: at com.skungee.bungeecord.BungeeSkungee.lambda$getPlayers$8(BungeeSkungee.java:163)
>.... [17:50:16 ERROR]: at com.skungee.bungeecord.BungeeSkungee$$Lambda$420.0000000068012FD0.apply(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.HashMap$KeySpliterator.forEachRemaining(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.ReferencePipeline.collect(Unknown Source)
>.... [17:50:16 ERROR]: at com.skungee.bungeecord.BungeeSkungee.getPlayers(BungeeSkungee.java:166)
>.... [17:50:16 ERROR]: at com.skungee.proxy.handlers.PlayerHandler.handle(PlayerHandler.java:63)
>.... [17:50:16 ERROR]: at com.sitrica.japson.server.SocketHandler.lambda$run$1(SocketHandler.java:68)
>.... [17:50:16 ERROR]: at com.sitrica.japson.server.SocketHandler$$Lambda$409.000000006800DC70.apply(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.ReferencePipeline$2$1.accept(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.HashMap$KeySpliterator.tryAdvance(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.ReferencePipeline.forEachWithCancel(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.AbstractPipeline.copyIntoWithCancel(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.FindOps$FindOp.evaluateSequential(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.stream.ReferencePipeline.findFirst(Unknown Source)
>.... [17:50:16 ERROR]: at com.sitrica.japson.server.SocketHandler.run(SocketHandler.java:70)
>.... [17:50:16 ERROR]: at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
>.... [17:50:16 ERROR]: at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
>.... [17:50:16 ERROR]: at java.lang.Thread.run(Unknown Source)

Steps to Reproduce the Problem

1. Recreate a normal Skungee instance
2. Disconnect then reconnect to the proxy

Specifications

Skungee Version: Skungee v2.0.0-ALPHA-7 (says 6 in console still)
Proxy Platform and Version: Waterfall-366 (proxy), latest build of Akarin 1.12.2 (backend)
System/Panel (if applicable): Eclipse OpenJ9 VM (build openj9-0.17.0, JRE 1.8.0 Linux amd64-64-Bit Compressed References 20191017_442 (JIT enabled, AOT enabled)

[Zulfen]

Issue is caused by the backend being disconnected.