is_a is not supported

[i582]

PHP Version: 7.4 and 8.0

OS: macOS

The minimal code in which the bug appears:

if (is_a($parent, 'Kirby\Cms\File') === true && $file = $parent->page()->file($path)) { ... }

Actual Behavior:

undefinedMethod: Call to undefined method {\Kirby\Cms\Model|null}->page()

Expected Behavior:

No error.