make terminals_available false if key is not in settings dict yet (#261)

Author: RRosio

nbclassic/tree/handlers.py

@@ -66,7 +66,7 @@ def get(self, path=''):
                 page_title=page_title,
                 notebook_path=path,
                 breadcrumbs=breadcrumbs,
-                terminals_available=self.settings['terminals_available'],
+                terminals_available=self.settings.get('terminals_available', False),
                 server_root=self.settings['server_root_dir'],
                 shutdown_button=self.settings.get('shutdown_button', False)
             ))