Guard 'content' against null in LoginLayout.onAttach

[coderabbitai]

Summary

In LoginLayout.onAttach, the content field is dereferenced at multiple points (appending to the element, setting the slot attribute, and executing JS) without a null check. If the layout attaches before routed content is available, this could throw a NullPointerException and potentially leave the overlay in a partially modified state.

Suggested fix

Add an early-return guard before the form replacement logic:

@Override
protected void onAttach(AttachEvent attachEvent) {
    super.onAttach(attachEvent);
    setOpened(true);
    if (content == null) {
        return;
    }
    // ... rest of the method
}

Context

This issue was identified during code review of PR #28 (comment: #28 (comment)). The concern originates from the original implementation and was not introduced by PR #28.

Reported by: @paodb