[Bukkit] Handle onDisable() better when onEnable() fails.

src/bukkit/java/com/sk89q/worldedit/bukkit/WorldEditPlugin.java

@@ -156,10 +156,12 @@ public class WorldEditPlugin extends JavaPlugin implements TabCompleter {
      */
     @Override
     public void onDisable() {
+        if (controller != null) {
             controller.clearSessions();
             controller.getPlatformManager().unregister(server);
             config.unload();
             server.unregisterCommands();
+        }
         this.getServer().getScheduler().cancelTasks(this);
     }