Hi Terry, this usually indicates a corrupted project file or a broken widget/layout definition. Mendix Modeler tries to load your project file (mpr file), which is stored in a BSON (Binary JSON) based format.
Error shows that the file structure is incomplete or corrupted, likely caused by a partial or faulty save, a problematic widget, or inconsistent model state. This prevents the Modeler from correctly parsing and opening your project.
1. Restore from Backup or Version Control, if you use Git or another VCS with Mendix Team Server, try reverting to a previous working commit/version before the issue appeared.
2. Delete/Replace Problematic Widget or Layout, the error points to a deeply nested widget inside a form. If you can identify and remove that widget from your source (maybe manually or via a text editor for the project files), it might help.
3. Use Mendix Support Tools, mendix support or community may have tools or scripts to repair corrupted project files or clean up the JSON structure.
4. Open a Copy in a Previous Mendix Version, sometimes opening the project in the older version of Mendix Modeler that last saved it can help, then export or fix the project before upgrading.
5. Contact Mendix Support, if none of the above works, provide the full error and project snapshot to Mendix Support for assistance.
Prevention Tips:
Commit frequently to version control.
Avoid closing the Modeler before saving and waiting for all background operations to finish.
Test new widgets or modules in isolated branches.
I hope this one helps you! :)