Error Number: 1048

Your error message says it all...
The bse_products_file schema says that the file_id column cannot be null.
The insert statement being executed does have null for file_id ...
INSERT INTO `bse_products_files` (`product_id`, `file_id`) VALUES (91, NULL)

Boom!

The root cause would appear to be that the "id" property of the row you find doesn't have a value, i.e. you are not finding a match for your query.