SettingsSidebar: null access when sidebar selection cleared

[ryonakano]

What Happened?

SettingsSidebar updates the visible child property to SettingsPage that SettingsSidebarRow has when the selection of SettingsSidebarRow is changed:

https://github.com/elementary/switchboard/blob/efd9be85b539f7ff816d5f1e24f1e478b3593e1f/lib/SettingsSidebar.vala#L99-L101

However, row could be null if the selection is cleared according to Valadoc, which causes a null access when you select some SettingsSidebarRow and removes it.

Steps to Reproduce

I made a test app to reproduce this issue easily.

1. Clone, build, and install https://github.com/ryonakano/junk/tree/settings-sidebar-bug
2. Open System Settings via Terminal
3. Go to "Bug in SettingsSidebar" plug under the Administration section
4. Click the + button in the action bar
5. Click the - button in the action bar
   
6. See the following message is shown in the terminal

(io.elementary.settings:5991): switchboard-CRITICAL **: 21:05:11.933: switchboard_settings_sidebar_row_get_page: assertion 'self != NULL' failed

(io.elementary.settings:5991): Gtk-CRITICAL **: 21:05:11.933: gtk_stack_set_visible_child: assertion 'GTK_IS_WIDGET (child)' failed

Expected Behavior

The above message are not shown and no null access happens.

OS Version

8.x (Circe)

Session Type

Classic Session (X11, This is the default)

Software Version

Compiled from git

Log Output

Hardware Info

No response