Fix errors reported by PHPStan #559

Job	Run time
Build	43s
Test PHP 8.3 (WP latest)	1m 59s
Lint JS & CSS	32s
Lint PHP & PHP Compatibility checks.	49s
Test PHP 8.3 (WP trunk)	1m 58s
Test PHP 8.3 (WP 6.3)	1m 58s
Test PHP 8.2 (WP latest)	1m 58s
Test PHP 8.2 (WP trunk)	2m 3s
Test PHP 8.2 (WP 6.3)	2m 0s
Test PHP 8.1 (WP latest)	1m 59s
Test PHP 8.1 (WP trunk)	2m 0s
Test PHP 8.1 (WP 6.3)	2m 0s
Test PHP 8.0 (WP latest)	2m 0s
Test PHP 8.0 (WP trunk)	2m 1s
Test PHP 8.0 (WP 6.3)	2m 6s
Test PHP 7.4 (WP latest)	1m 59s
Test PHP 7.4 (WP trunk)	1m 59s
Test PHP 7.4 (WP 6.3)	1m 58s
Test PHP 7.3 (WP latest)	1m 56s
Test PHP 7.3 (WP trunk)	1m 56s
Test PHP 7.3 (WP 6.3)	2m 3s
Test PHP 7.2 (WP latest)	1m 59s
Test PHP 7.2 (WP trunk)	2m 1s
Test PHP 7.2 (WP 6.3)	2m 2s
	43m 59s

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