Upgrade from 5.6 to 5.7 - Problem with Migration Tools

Hello everyone.
For upgrade from 5.6 to 5.7 then over, I just followed this guide: Tutorial upgrade from 5.6 to 5.7

All perfect until when I click on the “Export Batch” in the Concrete5
The add-on migration tool legacy broke with this message:
Fatal error: Call to a member function getValue() on null in ./concrete/core/libraries/attribute/controller.php on line 46

PHP: 5.6.40
Migration Tool (0.9.8)
Cache Settings: Off

If someone solve, there is the ISSUE in GitHub: GitHub ISSUE

Thank you in advance.

There is no one with a solution? Please need to solve this topic. Thank you in advance.

We need a solution, someone could help us? Thank you

I just tried this on one of my sites and created a batch file with all three components, Users, Pages and Stacks.
The export of the batch XML file went through without any problem.
So I can only assume you have a badly formed User or Page attribute causing your error.
Without looking at the files and database it is difficult to offer any more help or advice.

There is a video on YouTube showing how to do the export.