--- i/pympress/builder.py
+++ w/pympress/builder.py
@@ -76,7 +76,7 @@ class Builder(Gtk.Builder):
         Args:
             a_widget (:class:`~GObject.Object`): an object built by the builder, usually a widget
         """
-        for str_prop in (prop.name for prop in a_widget.props if prop.value_type == GObject.TYPE_STRING):
+        for str_prop in (prop.name for prop in list(a_widget.props) if prop.value_type == GObject.TYPE_STRING):
             try:
                 str_val = getattr(a_widget.props, str_prop)
                 if str_val: