When the inspector is opened closed and opened again in the demo it crashes.
To replicate this issue clone the source code and build the demo. Then open the inspector via the dropdown menu then close the inspector and open the inspector via the dropdown again.
The demo should then stop responding and an "Adwaita Demo" is not responding message will open soon after.
On the first opening of the inspector this was logged:
(adwaita-1-demo:194172): GLib-GObject-WARNING **: 17:00:31.194: cannot register existing type 'AdwInspectorPage'
(adwaita-1-demo:194172): GLib-CRITICAL **: 17:00:31.194: g_once_init_leave: assertion 'result != 0' failed
(adwaita-1-demo:194172): GLib-GIO-WARNING **: 17:00:31.194: Tried to register an extension of the type (null) to extension point gtk-inspector-page. Expected type is GtkWidget.
Nothing was logged on the second opening and the crash.