software-properties-gtk crashes if APT::Periodic::Upgrade-Package-Lists="3"

[Wes Waugh]

Changing `APT::Periodic::Update-Package-Lists` to a value not in the data model `model_updates_interval` in `/usr/share/software-properties/gtkbuilder/main.ui` calls an invalid method, crashing the program.

```
$ cat /etc/apt/apt.conf.d/10periodic
APT::Periodic::Update-Package-Lists "3";
APT::Periodic::Download-Upgradeable-Packages "1";
APT::Periodic::AutocleanInterval "0";
APT::Periodic::Unattended-Upgrade "0";
$ software-properties-gtk
Traceback (most recent call last):
  File "/usr/bin/software-properties-gtk", line 100, in <module>
    app = SoftwarePropertiesGtk(datadir=options.data_dir, options=options, file=file)
  File "/usr/lib/python3/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py", line 231, in __init__
    self.init_auto_update()
  File "/usr/lib/python3/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py", line 310, in init_auto_update
    self.combobox_update_interval.append_text(_("Every %s days")
AttributeError: 'ComboBox' object has no attribute 'append_text'
```

Occurs in:
```
$ lsb_release -rd
Description: Ubuntu 21.04
Release: 21.04
$ apt-cache policy software-properties-gtk
software-properties-gtk:
  Installed: 0.99.10.2
  Candidate: 0.99.10.2
  Version table:
 *** 0.99.10.2 500
        500 http://us.archive.ubuntu.com/ubuntu hirsute-updates/main amd64 Packages
        500 http://us.archive.ubuntu.com/ubuntu hirsute-updates/main i386 Packages
        100 /var/lib/dpkg/status
     0.99.10 500
        500 http://us.archive.ubuntu.com/ubuntu hirsute/main amd64 Packages
        500 http://us.archive.ubuntu.com/ubuntu hirsute/main i386 Packages

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.