Warning: Undefined array key 0 when with isset also being used

I am generating PDF from HTML page using Mpdf, My PHP version is 8.1 and Mpdf version is 8.1.2, I am using isset in the code as shown here:

if( isset($this->mpdf->table[$this->mpdf->tableLevel][$this->mpdf->tbctr[$this->mpdf->tableLevel]]['nr']))

I am getting this error message

Warning: Undefined array key 0 in C:\wamp\vendor\mpdf\mpdf\src\Tag\Table.php on line 589

What can be reason?