The signature given in gtk+-3.0.vapi (version 0.32) is:

public virtual unowned Gtk.CellEditable? start_editing (Gdk.Event event, Gtk.Widget widget, string path, Gdk.Rectangle background_area, Gdk.Rectangle cell_area, Gtk.CellRendererState flags);

In the source code gtkcellarea.c there are these lines:

 /* Activate or Edit the cell
       *
       * Currently just not sending an event, renderers afaics dont use
       * the event argument anyway, worst case is we can synthesize one.
       */
      if (gtk_cell_area_activate_cell (area, widget, activate_cell, NULL,
                                       &renderer_area, flags))
        return TRUE;

This means renderers can receive null events when activated.  I believe the correct signature is therefore:

public virtual unowned Gtk.CellEditable? start_editing (Gdk.Event? event, Gtk.Widget widget, string path, Gdk.Rectangle background_area, Gdk.Rectangle cell_area, Gtk.CellRendererState flags);

I would be grateful if this could be fixed, thanks.

(An alternative would be to ensure renderers are not sent null events)