syntax error, unexpected '[' on single sites

Archiver works fine on Multisite when Network Enabled. I tried installing on an independent single site. Activation failed with the error:
syntax error, unexpected '[' in [...] wp-content/plugins/archiver/includes/class-archiver.php on line 329

The same thing happened on 2 different independent single sites.

I located line 329 which is:

wp_send_json_error( $response->get_error_codes()[0] . ': ' . $response->get_error_messages()[0] );

I removed the 2 occurrences of [0], and plugin activated OK.

I don’t know enough about coding to understand what is going on here. But, with this change, Archiver functions as I expect it to.

It’s a great plugin. This is a very minor issue.

https://ww.wp.xz.cn/plugins/archiver/