I've followed the guide to save a custom object, when using OTGEdit convert tool I've been prompted with this error dialog (https://pastebin.com/D7LmDGad), then OTG edit went into an endless loading and never completed the conversion.

Here is the schematic I tried to convert: tree.zip

I've also tried to convert it using BulkEdit and apparently the conversion went fine
tree_Bo3.zip

Edit: there is a lot of AIR 😄 I've messed up with the settings apparently.
The REMOVE_Block: AIR and CENTER_Block: DIAMOND_BLOCK from the BulkEdit Readme and Settings are wrong (outdated maybe?) as I've found out that I had to replace them with REMOVE_Block: AIR:0 and CENTER_Block: DIAMOND_BLOCK:0.
Maybe the error in OTGedit is caused by something related?
Chatted directly with Lan and solved the AIR issue.