[plugin_installer] Use get_notebook() to access notebook

plugin_installer/plugin_installer.py

@@ -244,7 +244,7 @@ class PluginInstaller(GajimPlugin):
 
     def _on_connect_plugin_window(self, plugin_window):
         self._available_page = AvailablePage(
-            self.local_file_path('installer.ui'), plugin_window)
+            self.local_file_path('installer.ui'), plugin_window.get_notebook())
         self._available_page.set_download_in_progress(
             self._download_in_progress)
         self._available_page.connect('download-plugins',

plugin_installer/widget.py

@@ -31,11 +31,11 @@ class Column(IntEnum):
 
 
 class AvailablePage(Observable):
-    def __init__(self, builder_path, plugin_window):
+    def __init__(self, builder_path, notebook):
         Observable.__init__(self)
         self._ui = get_builder(builder_path)
 
-        self._notebook = plugin_window.plugins_notebook
+        self._notebook = notebook
         self._page_num = self._notebook.append_page(
             self._ui.available_plugins_box,
             Gtk.Label.new(_('Available')))