It's a bug in PHP 7.3.0 itself, see
getgrav/grav#2292
if you're running PHP 7.3.0 and you have opcode cache enabled, you need to add the following line to your php.ini:
opcache.optimization_level=0x7FFFBBFF
There is an opcache bug in PHP 7.3.0 which will be fixed on the next PHP version:
bugs.php.net/bug.php?id=77275