Unable to generate PDF 2020 R2: Cannot import a null node

[klees]

I'm running Flare version 2020 R2 and I just started getting this error when I create a PDF with track changes preserved.
...
Linking cross-references...
Creating chapters...
Build failed: Cannot import a null node.

I tried removing the Analyzer and the Output folders for my project.
I removed the AppData\Local\MadCap Software\MadCap Flare 2020 r2 folder.
I rebooted my system.

I keep getting this error when I try to run with Preserve track changes enabled. If I turn that off, the PDF generates fine.

Can anyone offer help with this?
Thanks
Karene

[klees]

The problem has disappeared after I accepted all review changes that were in the first file of my document. It's a mystery, but no longer an issue for me.