Fix NPE on inventory view & modernize code

[Pablete1234]

It appears if the user is viewing a non-player inventory, and it disapears (assuming a chest that is broken, or another entity that dies) it will keep on posting this NPE for the rest of the match, once per second.

[17:31:45 WARN]: java.lang.NullPointerException                                                                                                                                                                   
[17:31:45 WARN]:        at tc.oc.pgm.inventory.ViewInventoryMatchModule.checkMonitoredInventories(ViewInventoryMatchModule.java:292)                                                                              
[17:31:45 WARN]:        at tc.oc.pgm.inventory.ViewInventoryMatchModule.checkAllMonitoredInventories(ViewInventoryMatchModule.java:100)                                                                           
[17:31:45 WARN]:        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)                                                                                                                
[17:31:45 WARN]:        at tc.oc.pgm.util.concurrent.TaskExecutorService$Task.run(TaskExecutorService.java:228)                                                                                                   
[17:31:45 WARN]:        at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftTask.run(CraftTask.java:64)                                                                                                              
[17:31:45 WARN]:        at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:412)                                                                                   
[17:31:45 WARN]:        at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:900)                                                                                                               
[17:31:45 WARN]:        at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:385)                                                                                                               
[17:31:45 WARN]:        at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:827)                                                                                                               
[17:31:45 WARN]:        at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:728)                                                                                                             
[17:31:45 WARN]:        at java.lang.Thread.run(Thread.java:750)     

The change addresses that, and also revamps the code, since it was still matching players by name!! (this code is predating the UUID change 😅).