Fixed error caused by not specifying a mob spawner type.

src/WorldEditListener.java

@@ -257,10 +257,14 @@ public class WorldEditListener extends PluginListener {
                 text[3] = args0.length > 4 ? args0[4] : "";
                 return new SignBlock(blockType.getID(), data, text);
             } else if (blockType == BlockType.MOB_SPAWNER) {
+                if (args0.length > 0) {
                     if (!ServerInterface.isValidMobType(args0[1])) {
                         throw new InvalidItemException(arg, "Unknown mob type '" + args0[1] + "'");
                     }
                     return new MobSpawnerBlock(data, args0[1]);
+                } else {
+                    return new MobSpawnerBlock(data, "Pig");
+                }
             }
 
             return new BaseBlock(blockType.getID(), data);