No normal font in MUI 5?

I got a bug report for my new TreeView component for MUIClass. It looked weird the size calculation of the Font totally fails. A little bit testing reveals that the function OpenMUIFont fails for fkNormal to get the normal Font size this never happens on my MUI 3.8 systems. But the reporter uses MUI 5.0 maybe that’s a bug of MUI 5.0 or a new „feature“.

I added a fail save that I use the default font if the OpenMUIFont fails, so that should work now.