Panic when referencing property i changed callback from conditional element

[ogoffart]

Here another similar problem 🙈

export component AppWindow inherits Window {

    out property <length> scene-width;

    HorizontalLayout {
        if true: Rectangle {
            changed width => {
                scene-width = self.width;
            }
        }
    }
}

WIth a different error message:

thread 'main' panicked at /home/runner/work/slint/slint/internal/core/model.rs:928:38:
already borrowed: BorrowMutError
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[Info  - 10:23:16 PM] Connection to server got closed. Server will restart.
true
The Slint Language Server crashed. This is a bug. Please open an issue on https://github.com/slint-ui/slint/issues
The Slint Language Server crashed. This is a bug. Please open an issue on https://github.com/slint-ui/slint/issues
The Slint Language Server crashed. This is a bug. Please open an issue on https://github.com/slint-ui/slint/issues

Slint 1.8.0 LSP in VSCode extension on Linux

Originally posted by @ubruhin in #6426 (comment)