Due to a problem with the Kobo server, some shelve were synced with null values in the names. This shouldn't happen, but adding some checking for this so that it doesn't break things.

src/calibre/devices/kobo/books.py

@@ -199,6 +199,11 @@ class KTCollectionsBookList(CollectionsBookList):
                                 ('series' in collection_attributes and
                                  book.get('series', None) == category):
                             is_series = True
+
+                    # The category should not be None, but, it has happened.
+                    if not category:
+                        continue
+
                     cat_name = category.strip(' ,')
 
                     if cat_name not in collections:

src/calibre/devices/kobo/driver.py

@@ -1537,7 +1537,11 @@ class KOBOTOUCH(KOBO):
                 return bookshelves
 
             cursor = connection.cursor()
-            query = "select ShelfName from ShelfContent where ContentId = ? and _IsDeleted = 'false'"
+            query = "select ShelfName "         \
+                    "from ShelfContent "        \
+                    "where ContentId = ? "      \
+                    "and _IsDeleted = 'false' " \
+                    "and ShelfName is not null"         # This should never be nulll, but it is protection against an error cause by a sync to the Kobo server
             values = (ContentID, )
             cursor.execute(query, values)
             for i, row in enumerate(cursor):