OrbTk "Hey OrbTk!" text not showing in example app exec

I am new to OrbTk and have the Rust/Cargo/OrbTk setup on Pop!_OS. I am working through the online book at: Hello OrbTk! - The OrbTk Book

I used the “cargo run” command in the folder of the Cargo.toml file and also by the Build command in my Sublime Text editor app. The example OrbTk app pops up, but in all cases the text “Hey OrbTk!” from the TextBlock is not inside the window. I toggled dark and light mode from Gnome Settings, but the text is still missing. I also ran the “minimal-example” app from the Github site and the text inside was still missing. The code is below:

use orbtk::prelude::*;

fn main() {
// use this only if you want to run it as web application.
orbtk::initialize();

Application::new()
    .window(|ctx| {
        Window::new()
            .title("OrbTk-Book - Chapter 1.2")
            .position((100.0, 100.0))
            .size(420.0, 140.0)
            .child(
                TextBlock::new()
                    .font_size(28)
                    .h_align("center")
                    .text("Hey OrbTk!")
                    .v_align("center")
                    .build(ctx)
            )
            .build(ctx)
    })
    .run();

}

Thanks for any advice.