Consider code like:
---
public class SwitchGuard {
    public static void main(String... args) {
        test(null);
    }
    private static void test(Object o) {
        switch (o) {
            case null, String s when s.isEmpty() -> System.err.println("OK.");
            default -> {}
        }
    }
}
---

when ran, this should print "OK.", but it fails with a NPE:
---
$ java --enable-preview SwitchGuard
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.isEmpty()" because "<local3>" is null
        at SwitchGuard.test(SwitchGuard.java:7)
        at SwitchGuard.main(SwitchGuard.java:3)
---