'pre_user_query' interfering with user export

'users' != $current_screen->id in your file $current_screen is not set therefore, you are getting warning.

You can pass custom var in your user query and check before running your code.

$blogusers = get_users( 'orderby=nicename&order=ASC&my_filter=1' );

Here we are passing my_filter=1 then later we can check in pre_user_query

global $wpdb, $current_screen;

$vars = $user_search->query_vars;

if (isset($current_screen->id) && 'users' != $current_screen->id) {
    return;
} else if (empty($vars['myfilter'])) {
    return;
}

However, loading WordPress in a external file is not a good thing. Instead of external file you can wrap the whole code in a function and hook it on some action where headers are not sent.

Example:

Create a form instead of link e.g.

<form action="" method="POST">
    <input type="hidden" name="myprefix_export_csv" value="1" />
    <input type="submit" value="download" />
</form>

Then check if your custom key is set, if it is then export CSV with your current code.

add_action('init', 'export_csv'); //you can use admin_init as well
function export_csv() {
    if (!empty($_POST['myprefix_export_csv'])) {

        //Send headers
        //Query users
        //print data

        exit();
    }
}

You can also set nonce in form if you think other users can tweak it.

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