Prevent possible traceback when selecting a row while ServiceBrowser is still loading.

src/disco.py

@@ -725,10 +725,12 @@ _('This type of service does not contain any items to browse.'))
 		self.travel(jid, '')
 
 	def on_services_treeview_row_activated(self, widget, path, col = 0):
-		self.browser.default_action()
+		if self.browser:
+			self.browser.default_action()
 
 	def on_services_treeview_selection_changed(self, widget):
-		self.browser.update_actions()
+		if self.browser:
+			self.browser.update_actions()
 
 
 class AgentBrowser: