If I open gnome-documents from a terminal and subsequently close it, I get this error:

(org.gnome.Documents:12708): Gjs-CRITICAL **: Attempting to call back into JSAPI during the sweeping phase of GC. This is most likely caused by not destroying a Clutter actor or Gtk+ widget with ::destroy signals connected, but can also be caused by using the destroy(), dispose(), or remove() vfuncs. Because it would crash the application, it has been blocked and the JS callback not invoked.

(org.gnome.Documents:12708): Gjs-CRITICAL **: The offending callback was destroy(), a vfunc.

I ran git bisect, which pointed to 026e25. This is the commit that added the destroy handler to the selection toolbar. Setting a breakpoint on the warning shows that it is generated when gtk_widget_dispose() is called on the selection toolbar.