diff --git a/gaphor/services/guimanager.py b/gaphor/services/guimanager.py
index c9aa953fe..94eccc1c3 100644
--- a/gaphor/services/guimanager.py
+++ b/gaphor/services/guimanager.py
@@ -11,7 +11,7 @@ class GUIManager(object):
     interface.implements(IService)
 
     def __init__(self):
-        pass
+        self._ui_components = dict()
 
     main_window = property(lambda s: s._main_window)
 
diff --git a/gaphor/ui/mainwindow.py b/gaphor/ui/mainwindow.py
index 67816dab5..03ceafca2 100644
--- a/gaphor/ui/mainwindow.py
+++ b/gaphor/ui/mainwindow.py
@@ -18,7 +18,7 @@ from gaphor.ui.menufactory import toolbox_to_menu
 from gaphor.ui.objectinspector import ObjectInspector
 
 
-from interfaces import IDiagramElementReceivedFocus
+from interfaces import IUIComponent, IDiagramElementReceivedFocus
 from gaphor.interfaces import IServiceEvent
 from zope import component
 
@@ -27,7 +27,7 @@ class MainWindow(AbstractWindow):
     The main window for the application.
     It contains a Namespace-based tree view and a menu and a statusbar.
     """
-    interface.implements(IService, IActionProvider)
+    interface.implements(IUIComponent, IActionProvider)
 
     properties = inject('properties')
     element_factory = inject('element_factory')