Fix 500 error

When the users logs out in one tab, then in another (previously loaded)
tab clicks on shared or personal.

Author: jdavid

caterva2/utils.py

@@ -50,11 +50,12 @@ def walk_files(root, exclude=None):
     if exclude is None:
         exclude = set()
 
-    for path in root.glob("**/*"):
-        if path.is_file():
-            relpath = path.relative_to(root)
-            if str(relpath) not in exclude:
-                yield path, relpath
+    if root is not None:
+        for path in root.glob("**/*"):
+            if path.is_file():
+                relpath = path.relative_to(root)
+                if str(relpath) not in exclude:
+                    yield path, relpath
 
 
 def iterdir(root):