This was working fine for me the other day, but today suddenly started crashing on startup. Tried a fresh install, no luck. I had been editing a custom script before opening, but undid the changes and also tried removing the script from the extensions folder but no luck there. The first crash was after opening a supertiled2unity prefab directly from my unity project.
Usually when an application crashes on macOS there is a crash dialog popping up, which could contain useful information about what triggers the crash. Do you see such a dialog?
Also, a fresh install does not reset your session or configuration. You could try deleting your .tiled-session file (stored alongside the project file) or resetting the settings by deleting ~/Library/Preferences/org.mapeditor.Tiled.plist. However, please see if you can help find the cause of the crash before doing these steps (or, backup the files instead of deleting them, in case something in them can help reproduce the crash).