Example of problem

The bug is totally unrelated to FLEXIcontent

It is a known joomla bug,
and it is not new to J3.7 it existed before, just it occured in a different file (the code producing it was moved to another file)

The com_content (legacy) router under some cases (the view variable matches, etc) tries to "examine" menu items that belong to other components !!

In the end these menu items are not selected but in the middle of the code they are examined as if they are com_content menu items
... and you get these notices

Quick solution: disable error reporting in global config you do not need / should not enable it in a live site

See a long discussion here
#6488

A bug that should have been fixed when that PR was still good
Still a simpler / easy fix is possible, with no risk for break (just for the sake of not changing the broken code) by using an isset statement on the id index of the menu

And to make it clear, it is always not a harmless bug, if a menu item of different component is found that has a matching number in the id then it will be selected, to be used for creating a com_content URL using a menu item of different component

The original patch is here: 64f5ac9 just the file is called now legacyrouter.php

@Vednier to check if that fixes the issue you may try to apply patch manually

@piotr-cz - yes, this little fix stop notices from appearing in ErrorLog. Thanks.

Closed as there is a PR for testing

I accidentally caused this error to display on my own site when I copied the url of an existing page on my site and used it in a menu item of type URL. I published the menu item and I removed the symptom.

I use Joomla 3.8.3 and I have the same problem. The problem appeared when I created many menu items of the type "Menu Item Alias" that point to other menu items of the type "Articles » Single Article".

I managed to fix the problem by changing the line 95 in "components\com_content\helpers\legacyrouter.php" from:

if ($menuItem !== null
&& $menuItem->query['view'] == $query['view']
&& isset($menuItem->query['id'], $query['id'])
&& $menuItem->query['id'] == (int) $query['id'])

to:

if ($menuItem !== null
&& isset($menuItem->query['view'])
&& $menuItem->query['view'] == $query['view']
&& isset($menuItem->query['id'], $query['id'])
&& $menuItem->query['id'] == (int) $query['id'])

Actually I added the line:
&& isset($menuItem->query['view'])

It seems that they forgot to check the existence of the variable $menuItem->query['view'].

@impressionestudio There is a closed PR #18757 to fix this. Please provide exact steps/instructions on how to reproduce it in v3.8.3 so that it can be tested/merged. Thanks.