I used the release tarball from the Selfoss homepage to set up a Selfoss installation on top of PHP 7.2.
This would not work because of this bug f3-factory/fatfree-core#238.

After updating the fatfree-core package using Composer Selfoss works with PHP 7.2.
The release tarball on the website should be updated so that Selfoss can be used with PHP 7.2. Thanks :-)