Fix potential NPE in vanish API. Fixes BUKKIT-1541

By: Travis Watkins <amaranth@ubuntu.com>

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java

@@ -596,6 +596,10 @@ public class CraftPlayer extends CraftHumanEntity implements Player {
         if (hiddenPlayers.containsKey(player.getName())) return;
         hiddenPlayers.put(player.getName(), player);
 
+        if (getHandle().netServerHandler == null) {
+            return;
+        }
+
         //remove this player from the hidden player's EntityTrackerEntry
         EntityTracker tracker = ((WorldServer) entity.world).tracker;
         EntityPlayer other = ((CraftPlayer) player).getHandle();
@@ -614,6 +618,10 @@ public class CraftPlayer extends CraftHumanEntity implements Player {
         if (!hiddenPlayers.containsKey(player.getName())) return;
         hiddenPlayers.remove(player.getName());
 
+        if (getHandle().netServerHandler == null) {
+            return;
+        }
+
         EntityTracker tracker = ((WorldServer) entity.world).tracker;
         EntityPlayer other = ((CraftPlayer) player).getHandle();
         EntityTrackerEntry entry = (EntityTrackerEntry) tracker.trackedEntities.get(other.id);