Backport PR #1871: Allow None for post_save_hook

See gh-1868

We're also discussing changes to traitlets that might avoid the need for this, in ipython/traitlets 342. But this is the simple fix.

Signed-off-by: Min RK <[email protected]>

Author: Carreau

notebook/services/contents/filemanager.py

@@ -83,7 +83,7 @@ def _save_script_changed(self):
 
         self.post_save_hook = _post_save_script
 
-    post_save_hook = Any(None, config=True,
+    post_save_hook = Any(None, config=True, allow_none=True,
         help="""Python callable or importstring thereof
 
         to be called on the path of a file just saved.