Could you list some examples of pages that cause this or a link to the logstash query?

https://logstash.wikimedia.org/app/kibana#/dashboard/Reading-Web?_g=(refreshInterval%3A(display%3AOff%2Cpause%3A!f%2Cvalue%3A0)%2Ctime%3A(from%3Anow-7d%2Cmode%3Aquick%2Cto%3Anow))

Change 436077 had a related patch set uploaded (by BearND; owner: BearND):
[mediawiki/extensions/MobileFrontend@master] Ensure pageprops are available before use

https://gerrit.wikimedia.org/r/436077

Change 436094 had a related patch set uploaded (by Pmiazga; owner: Pmiazga):
[mediawiki/extensions/MobileFrontend@master] Verify that data['pageprops'] exists before using it

https://gerrit.wikimedia.org/r/436094

Bringing to sprint as it's polluting our logs. Another example of just do it task. Oh, now I see that @bearND also made a patch.

@bearND could you check my patch: https://gerrit.wikimedia.org/r/436094 -> it verifies that key exists and also verifies that this is an array. I'm sorry, I didn't notice you also fixed it.
The patch submitted by you solves Notice: Undefined index: pageprops error, but

Warning: Invalid operand type was used: array_key_exists expects an array or an object; false returned

can still occur when $data['pageprops'] is something other than array (false for example).

Change 436077 abandoned by BearND:
Ensure pageprops are available before use

Reason:
in favor of I4ece5a01425

https://gerrit.wikimedia.org/r/436077

@pmiazga Your patch looks better to me. Thanks!

Change 436094 merged by jenkins-bot:
[mediawiki/extensions/MobileFrontend@master] Verify that data['pageprops'] exists before using it

https://gerrit.wikimedia.org/r/436094

This can skip QA. It will be live next week. We'll want to check that logstash is clean again. Alternatively we can resolve this now and lean on chores to bubble this up if it's a problem!