Check if $header and $footer are not null

[vpecinka]

Hi,

I found the following problem in the Nextcloud project, where this code is used in Forms appliaction. It crashes for me because $header and $footer variables are null.

I sugest to fix the code with isset() test here:

• PhpSpreadsheet/src/PhpSpreadsheet/Reader/Ods/PageSettings.php

  Line 69 in fcf03b9

  $headerProperties = $header->getElementsByTagNameNS($this->stylesNs, 'header-footer-properties')[0];

• PhpSpreadsheet/src/PhpSpreadsheet/Reader/Ods/PageSettings.php

  Line 72 in fcf03b9

  $footerProperties = $footer->getElementsByTagNameNS($this->stylesNs, 'header-footer-properties')[0];

The safe code should look like: $headerProperties = isset($header) ? $header->getElementsByTagNameNS($this->stylesNs, 'header-footer-properties')[0] : null;