Further fix delete books (#119)

cps/web.py

@@ -1302,6 +1302,7 @@ def delete_book(book_id):
             for c in cc:
                 cc_string = "custom_column_" + str(c.id)
                 if not c.is_multiple:
+                    if len(getattr(book, cc_string)) > 0:
                         if c.datatype == 'bool':
                             del_cc = getattr(book, cc_string)[0]
                             getattr(book, cc_string).remove(del_cc)