Kubuntu installer does not offer to select file system type when disc encryption is selected

Release: 18:10

Different to the Ubuntu 18.10 installer the Kubuntu 18.10 installer does not support selecting a file system type when creating an encrypted root partition during installation.
The consequence is that one will always end up with luks+ext4.

My specific use case is that I do need full disk encryption on my laptop because I am holding my customers personal information. But I would also like to benefit from the capabilities of a modern filesystem = btrfs.

Workarounds considered:
- Walk through the standard installation and convert from ext4 to btrfs post installation. Won't work any longer as btrfs-convert has been pulled.
- Install Ubuntu first and manually install the KDE environment afterwards. This works, but leaves lots of Gnome stuff behind and somehow defeats the purpose of having a separate distribution.
- Tried to create the encrypted partition + btrfs outside the installer and have the Kubuntu installer use the existing ones. Couldn't get this to work as there is no way to open the container during installation. Opening the container with KDE-Partition manager before launching the installer should work in principle but made the installer crash.