fix(niri/workspaces): segfault when setting workspace button markup

[jaschiu]

Hi, waybar was crashing for me when I was using the niri/workspaces module. My LLM wrote the below description for me and debugged it for me. I am not sure if this fix needs to also be applied to other modules using the same pattern such as sway/workspaces

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() 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).