file_transfer_t *imcb_file_send_start(struct im_connection *ic, char *handle, char *file_name, size_t file_size)
{
 	bee_t *bee = ic->bee;
 	bee_user_t *bu = bee_user_by_handle(bee, ic, handle);
 
	if (bee->ui->ft_in_start && bu) {
 		return bee->ui->ft_in_start(bee, bu, file_name, file_size);
 	} else {
 		return NULL;
	}
}

The product dereferences a pointer that it expects to be valid but is NULL.