get GtkWindow instance when we use set_transient_for. Fixes #2170

src/advanced.py

@@ -46,7 +46,8 @@ class AdvancedConfigurationWindow:
 	def __init__(self):
 		self.xml = gtkgui_helpers.get_glade('advanced_configuration_window.glade')
 		self.window = self.xml.get_widget('advanced_configuration_window')
-		self.window.set_transient_for(gajim.interface.instances['preferences'])
+		self.window.set_transient_for(
+			gajim.interface.instances['preferences'].window)
 		self.entry = self.xml.get_widget('advanced_entry')
 		self.desc_label = self.xml.get_widget('advanced_desc_label')
 		self.restart_label = self.xml.get_widget('restart_label')