Message: Trying to access array offset on value of type null

I'm getting this error on multiple occasion in a script (invoiceplane) I have been using for a few years now but which hasn't been maintained unfortunately by its creators:

Message: Trying to access array offset on value of type null

My server has been upgrade to PHP 7.4 and I'm looking for a way to fix the issues and maintain the script myself since I'm very happy with it.

This is what's on the line that gives the error:

$len = $cOTLdata['char_data'] === null ? 0 : count($cOTLdata['char_data']);

$cOTLdata is passed to the function:

public function trimOTLdata(&$cOTLdata, $Left = true, $Right = true)
{
	$len = $cOTLdata['char_data'] === null ? 0 : count($cOTLdata['char_data']);
	$nLeft = 0;
	$nRight = 0;
    //etc

It's included in mpdf btw, but simply overwriting the files from the github repository did not fix the errors.

This happens because $cOTLdata is not null but the index 'char_data' does not exist. Previous versions of PHP may have been less strict on such mistakes and silently swallowed the error / notice while 7.4 does not do this anymore.

To check whether the index exists or not you can use isset():

isset($cOTLdata['char_data'])

Which means the line should look something like this:

$len = isset($cOTLdata['char_data']) ? count($cOTLdata['char_data']) : 0;

Note I switched the then and else cases of the ternary operator since === null is essentially what isset already does (but in the positive case).