I'm currently in the process of overhauling the Inventory API contribution. I believe that is a bug I found a fixed a few weeks ago. I haven't released a new version yet. Until then, the error should only show up when you delete nodes, and no harm is being done by the bug so you you can just ignore the error. (I've put your error message on the list of things to check before I post the new version, just to make sure it really is fixed).