Runtime Error / Location: Universal/REALstring.cpp:130 Condition: usageCount

Runtime Error

Location: Universal/REALstring.cpp:130
Condition: usageCount
[OK]

This is what I get when I load the source of my Fonts Manager app into any recent version of Xojo.

The app was created with 2016R3. When I open in 2016R3, no error message.

I tried to save in XML format, no difference.

Any ideas ?