Have a look at the propertree GitHub page, it has the solution.....
ProperTree crashes on Big Sur (macOS 11)
This is due to the default python installs on macOS leveraging an older tk version - which lacks support for macOS 11. To solve this, you can download and install the latest python 3 version from https://www.python.org/downloads/mac-osx/then leverage the buildapp-python3.command from ProperTree's Scripts directory to build a .app bundle that will leverage that python version.
If you already have python 3 installed via brew or another package manager - it is likely still linking to the system tkversion, which will still have issues unless linked against a newer version.