Unable to use Swift function that returns String on Java side

I'm using jextract-swift to create Java Classes for the Sources/MySwiftLibrary/SwiftKitSamleApp for the sample SwiftKitSampleApp.

I created the following function just to test something in HelloJava2Swift.swift

public func retrieveToken() -> String {
  return "test"
}

But when I try to do the following in MySwiftLibraryTest.java

@Test
    void call_retrieveToken() {
        MySwiftLibrary.retrieveToken();
        assertNotNull(MySwiftLibrary.retrieveToken$address());
    }

It gives the following error

I'm probably forgetting something important.

Start with the retrieveToken declaration in Sources/MySwiftLibrary/HelloJava2Swift.swift and the generated call in MySwiftLibraryTest.java. Reproduce the failure using the jextract-swift-generated MySwiftLibrary classes, then determine what prevents a Swift String return from being called on the Java side. Done means the Java test can call retrieveToken() and assert that retrieveToken$address() is not null.