Fix starting IPython in vi editing mode

closes  #11404

IPython/terminal/interactiveshell.py

@@ -147,7 +147,8 b' class TerminalInteractiveShell(InteractiveShell):'
     @observe('editing_mode')
     def _editing_mode(self, change):
         u_mode = change.new.upper()
-        self.pt_app.editing_mode = u_mode
+        if self.pt_app:
+            self.pt_app.editing_mode = u_mode
 
     @observe('highlighting_style')
     @observe('colors')