Idk how it can be null, but well ...

src/main/java/fr/xephi/authme/cache/limbo/LimboCache.java

@@ -117,6 +117,8 @@ public class LimboCache {
      * @param name String
      */
     public void deleteLimboPlayer(String name) {
+    	if (name == null)
+    		return;
         cache.remove(name);
     }
 
@@ -128,6 +130,8 @@ public class LimboCache {
      * @return LimboPlayer
      */
     public LimboPlayer getLimboPlayer(String name) {
+    	if (name == null)
+    		return null;
         return cache.get(name);
     }
 
@@ -139,6 +143,8 @@ public class LimboCache {
      * @return boolean
      */
     public boolean hasLimboPlayer(String name) {
+    	if (name == null)
+    		return false;
         return cache.containsKey(name);
     }
 

src/main/java/fr/xephi/authme/process/join/AsynchronousJoin.java

@@ -253,8 +253,11 @@ public class AsynchronousJoin {
                 ? m.retrieve(MessageKey.REGISTER_EMAIL_MESSAGE)
                 : m.retrieve(MessageKey.REGISTER_MESSAGE);
         }
-        BukkitTask msgTask = sched.runTaskAsynchronously(plugin, new MessageTask(plugin, name, msg, msgInterval));
-        LimboCache.getInstance().getLimboPlayer(name).setMessageTaskId(msgTask);
+        if (LimboCache.getInstance().getLimboPlayer(name) != null)
+        {
+            BukkitTask msgTask = sched.runTaskAsynchronously(plugin, new MessageTask(plugin, name, msg, msgInterval));
+            LimboCache.getInstance().getLimboPlayer(name).setMessageTaskId(msgTask);
+        }
     }
 
     private boolean needFirstSpawn() {