Created attachment 190273 [details]
The backtrace

This program:

import gobject

class X(gobject.GObject):
    __gproperties__ = {
        'foo' : (gobject.TYPE_OBJECT, 'foo property', None,
                 gobject.PARAM_READWRITE)
    }

    def __init__(self):
        gobject.GObject.__init__(self)
        self.foo = None

    def do_get_property(self, prop):
        if prop.name == 'foo':
            return self.foo

    def do_set_property(self, prop, value):
        if prop.name == 'foo':
            self.foo == value

gobject.type_register(X)

x = X()
x.props.foo = None

will give the following warning:

GLib-GObject-CRITICAL **: g_object_set_qdata: assertion `G_IS_OBJECT (object)' failed

This is with glib 2.28.8, pygobject 2.28.4 and Python 2.7. The backtrace is
attached. It seems to me that pyg_value_as_pyobject should check that the
return value of g_value_get_object is not NULL before passing it to
pygobject_new_sunk.