call-window: Reference selector item widget in creation callback

The return value of this callback, used for the call selector flow
box, should have a reference added to it.  The callback's return value
is annotated as "transfer full".

src/calls-call-window.c

@@ -110,7 +110,10 @@ static GtkWidget *
 call_holders_create_widget_cb (CallsCallHolder *holder,
                                CallsCallWindow *self)
 {
-  return GTK_WIDGET (calls_call_holder_get_selector_item (holder));
+  CallsCallSelectorItem *item =
+    calls_call_holder_get_selector_item (holder);
+  g_object_ref (G_OBJECT (item));
+  return GTK_WIDGET (item);
 }