### Problem
Waybar segfaults on startup when `niri/workspaces` is the only configured
module (default options). Right after the first `WorkspacesChanged` event:
Thread 1 "waybar" received signal SIGSEGV
#0 gtk_label_set_markup ()
#1 waybar::modules::niri::Workspaces::doUpdate () at src/modules/niri/workspaces.cpp:106
### Root cause
[doUpdate()](cci:1://file:///home/lj/Downloads/Waybar/src/modules/niri/window.cpp:27:0-90:1) set the label markup via:
static_cast<Gtk::Label*>(button.get_children()[0])->set_markup(name);
The button's child *is* a valid `GtkLabel`, but gtkmm's `get_children()`
returns it wrapped as a generic `Gtk::Widget` (confirmed: `dynamic_cast`
to `Gtk::Label*` yields `nullptr`). The unchecked `static_cast` then
performs an invalid downcast, producing a corrupt pointer whose `gobj()`
is garbage (`0x1`), so `gtk_label_set_markup()` dereferences it and crashes.
This is reliably triggered when no other module has instantiated a
`Gtk::Label` yet (so the `Gtk::Label` wrapper isn't registered), which is
exactly the case for a `niri/workspaces`-only bar. The same idiom exists in
`sway/workspaces` and `wayfire/workspaces`; it's masked there because typical
configs include other label-using modules.
### Testing
- Reproduced the crash on 0.15.0 / current `master` with a minimal
`"modules-left": ["niri/workspaces"]` config under niri.
- After the fix: no crash; three workspace buttons render with correct
names/labels (`niri-workspace-1/2/3`).
Jason Chiu