Skin is Material Dark and needs to be in C:\AlphaSkins\Skins
Run the app and press enter to get the exception.
FreeAndNil is always better than Free – if you do end up using it later you can tell it is a nil pointer use rather than an arbitary address (or worse it works and corrupts some other object). It also means the exception is in the the deleted object not several levels down.
I have attached a reworked exe and full project.
It should now need less bpls – hopefully only Berlin ones.
To see the issue run the exe and select the checkbox, then press enter.
To see it work correctly click the apply button.
They both call PopulateEditor that destroys the TsCheckBox.