Hello, we've just migrated the data from an old version of Mediawiki / an unsupported OS platform to MediaWIki 1.32.0 / running on PHP 7.0 on Ubuntu 16.04. The migrate was done by building a new platform, copying the SQL data (and image files) from the old system and making minimum necessary changes to the new plarform's configuration file. The data import worked fine, migration script (php /usr/share/mediawiki/maintenance/update.php) completed without error, wiki 'mostly' works. The only error is as in the subject when attempting to preview a change (or actually, preview itself, you don't need to change anything, just ask for a preview). These fail with message of the form:
[04f796e3b179a4cdf7946122] 2019-05-20 15:11:35: Fatal exception of type "Error"
with a different number in the square brackets and time stamp.
The web server (nginx 1.16.0) records error messages of the form below, one error message for each attempt to preview:
2019/05/20 15:24:06 [error] 18426#18426: *8136 FastCGI sent in stderr: "PHP message: PHP Notice: Undefined property: CKeditorEditPage::$isCssJsSubpage in /usr/share/mediawiki/extensions/WYSIWYG-src/WYSIWYG/CKeditorEditPage.body.php on line 35" while reading response header from upstream, client: [redacted], server: [redacted].com, request: "POST /index.php?title=Category:Sysadmin&action=submit HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.0-fpm.sock:", host: "[redacted].com", referrer: "https://[redacted].com/index.php?title=Category:Sysadmin&action=edit"
where I've removed the external address of the site and the server name and replaced then with [redacted] instead.
Saving changes *without* preview works fine, the saved pages are rendered correctly. WIth preview it consistently fails with the error message.