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) {
		return bee->ui->ft_in_start(bee, bu, file_name, file_size);
	} else {
		return NULL;
	}
}
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.