Bug#985551: sddm: minor error during upgrade while another dm is the default

[Patrice DUROUX <duroux.patrice@orange.fr>]

Package: sddm
Version: 0.19.0-3
Severity: normal

Dear Maintainer,

During last upgrade of a Sid system:

Paramétrage de sddm (0.19.0-3) ...
Installation de la nouvelle version du fichier de configuration /etc/sddm/Xsession ...
Installation de la nouvelle version du fichier de configuration /etc/sddm/wayland-session ...
Failed to preset unit: File /etc/systemd/system/display-manager.service already exists and is a symlink to /lib/systemd/system/lightdm.service.
/usr/bin/deb-systemd-helper: error: systemctl preset failed on sddm.service: No such file or directory

Sorry for the messages in French, this is the default locale for this
system.

Thanks,
Patrice

-- System Information:
Debian Release: bullseye/sid
  APT prefers unstable
  APT policy: (500, 'unstable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 5.9.0-4-amd64 (SMP w/24 CPU threads)
Kernel taint flags: TAINT_FIRMWARE_WORKAROUND
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages sddm depends on:
ii  adduser                   3.118
ii  debconf [debconf-2.0]     1.5.75
ii  libc6                     2.31-9
ii  libgcc-s1                 10.2.1-6
ii  libpam0g                  1.4.0-7
ii  libqt5core5a              5.15.2+dfsg-5
ii  libqt5dbus5               5.15.2+dfsg-5
ii  libqt5gui5                5.15.2+dfsg-5
ii  libqt5network5            5.15.2+dfsg-5
ii  libqt5qml5                5.15.2+dfsg-4
ii  libqt5quick5              5.15.2+dfsg-4
ii  libstdc++6                10.2.1-6
ii  libsystemd0               247.3-3
ii  libxcb-xkb1               1.14-3
ii  libxcb1                   1.14-3
ii  qml-module-qtquick2       5.15.2+dfsg-4
ii  tightvncserver [xserver]  1:1.3.10-2
ii  x11-common                1:7.7+22
ii  xauth                     1:1.1-1
ii  xserver-xephyr [xserver]  2:1.20.10-3
ii  xserver-xorg [xserver]    1:7.7+22
ii  xvfb [xserver]            2:1.20.10-3

Versions of packages sddm recommends:
pn  haveged                              <none>
ii  libpam-systemd                       247.3-3
ii  sddm-theme-debian-maui [sddm-theme]  0.19.0-3

Versions of packages sddm suggests:
ii  libpam-kwallet5           5.20.5-1
pn  qtvirtualkeyboard-plugin  <none>

-- debconf information:
  sddm/daemon_name: /usr/bin/sddm
* shared/default-x-display-manager: lightdm