Warning: trying to access array offset on value of type null in

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PHP

WordPress

I am learning making website for first time.
I installed XAMPP and installed WP.
I installed a theme I purchased several years ago.

I am unable to view the theme properly in my local host. The error I get is

Warning: Trying to access array offset on value of type null in Line...
Following are the lines of code with error: -

File name : Top bar
Line 4 with error

<?php global $adforest_theme; ?>
<?php
$msg_count = 0;
////LINE 4 with error ////  if (!$adforest_theme['sb_top_bar']) {
    return;
}
?>

File name : sitelogo
Line 6 with error

<?php global $adforest_theme; ?>
<a href="<?php echo home_url('/'); ?>">
    <?php
    
    $transparent_flag = false;
///Line 6 //// if ($adforest_theme['sb_header'] == 'transparent-2' || $adforest_theme['sb_header'] == 'transparent-3' || $adforest_theme['sb_header'] == 'modern' || $adforest_theme['sb_header'] == 'transparent') {
     $transparent_flag = true;  
    }

File name : mainnav.php
Line 6 with error

<?php

global $adforest_theme;

$menu_ul_flag = true;
////line 6 //// if ($adforest_theme['sb_header'] == 'modern2' || $adforest_theme['sb_header'] == 'modern3') {
    $menu_ul_flag = false;
}

File name : blogloop.php
Line 23 with error

<?php
                        $no_img  =	'no-img';
                        $response	=	adforest_get_feature_image( get_the_ID(), 'adforest-category' );
  ////line 23///       if( $response[0] != "" )
                        {
                            $no_img  =	'';
                    ?>

What I have tried:

i dont know PHP programming therefore I did not try to alter the code. I have some experience with OOP which I learnt in class 12 in 2006. I think technology must have changed greatlys ince then.