[4.0] Active menu does not exist, fatal error

[sakiss]

Pull Request for Issue # .#30812
Follow up of PR: #31100

Summary of Changes

$app->getMenu()->getActive();
can return a MenuItem or null

by calling a function when null is returned, it generates a fatal error.

Testing Instructions

Add a dummy/non existent ItemId in your non sef urls.
E.g. /index.php?option=com_content&view=article&id=2:another-article&catid=9&itemId=5000
In that case there is no menu item with id 5000

Actual result BEFORE applying this Pull Request

Expected result AFTER applying this Pull Request

No fatal error

Documentation Changes Required

No

[ghost]

I have tested this item ✅ successfully on d8568dd

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[laoneo]

I have tested this item ✅ successfully on d8568dd

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/31156.}

[richard67]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/31156.}