If you take a look at tracktion_SelectableClass.h you’ll see the interface you need to create for each type of class you want to be able to select.
Once you have a subclass implemented for each type of Selectable you want to select, you can register it globally with the macro DECLARE_SELECTABLE_CLASS. E.g.
class WaveAudioClipSelectableClass : public SelectableClass
{
public:
// Implement the methods you need
};
DECLARE_SELECTABLE_CLASS (WaveAudioClip)
But as @RolandMR says, I think we just need to avoid the assertion in your case?