Awesome building!!
The problem is, that you maybe used the "item" command in combination with an invalid item? You need to edit the world file (inside the gamefolder/Worlds) manually (e.g. with a program like navicat), go to the table "Inventory" and manually delete the slot "Inventory" for your player.
Alternatively, you can zip your world and send it to support@jiw-games.net, then I can fix that for you