Allow just one edit instance per resource

core/src/main/java/io/github/dre2n/dungeonsxl/command/EditCommand.java

@@ -58,6 +58,11 @@ public class EditCommand extends BRCommand {
         }
 
         DResourceWorld resource = worlds.getResourceByName(mapName);
+        if (resource == null) {
+            MessageUtil.sendMessage(sender, DMessages.ERROR_NO_SUCH_MAP.getMessage(mapName));
+            return;
+        }
+
         DEditWorld editWorld = resource.instantiateAsEditWorld();
         DGroup dGroup = DGroup.getByPlayer(player);
         DGlobalPlayer dPlayer = plugin.getDPlayers().getByPlayer(player);

core/src/main/java/io/github/dre2n/dungeonsxl/world/DResourceWorld.java

@@ -236,14 +236,20 @@ public class DResourceWorld {
     }
 
     /**
-     * @return an instance of this world
+     * @return an old or a new instance of this world.
      */
     public DEditWorld instantiateAsEditWorld() {
+        for (DEditWorld instance : worlds.getEditWorlds()) {
+            if (instance.getName().equals(getName())) {
+                return instance;
+            }
+        }
+
         return (DEditWorld) instantiate(false);
     }
 
     /**
-     * @return an instance of this world
+     * @return a new instance of this world
      */
     public DGameWorld instantiateAsGameWorld() {
         return (DGameWorld) instantiate(true);