Guard against null messages in Messenger. Fixes #391

src/main/java/com/garbagemule/MobArena/Messenger.java

@@ -16,7 +16,7 @@ public class Messenger
 
     public boolean tell(CommandSender p, String msg) {
         // If the input sender is null or the string is empty, return.
-        if (p == null || msg.equals("")) {
+        if (p == null || msg == null || msg.equals("")) {
             return false;
         }