Description:
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The documentation for the DateTime constructor says that when using the $timezone parameter, the first parameter may be null [1].
Enabling strict types, an exception is thrown [2].

[1] http://php.net/manual/en/datetime.construct.php#refsect1-datetime.construct-parameters
[2] https://3v4l.org/DKdFT

Test script:
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<?php
declare(strict_types=1);
var_dump((new DateTime(null, new DateTimeZone('Europe/Berlin')))->format('d-m-Y H:i'));

Expected result:
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Current time

Actual result:
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Fatal error: Uncaught TypeError: DateTime::__construct() expects parameter 1 to be string, null given in line 3