Widgets created in async functions do not appear

### Problem description

When we append a new view to the `contentView` within `async` function, it is not rendered. However, it appears when we move the app to the background and then come back to the foreground. We can reproduce the issue with the following snippet, which new screen (with red background) should be opened after 4 seconds when we click the button on the first yellow screen.

### Expected behavior

The rendering of the view should not be affected by any `async` functions.

### Environment

- Tabris.js version: 3.7.2
- Device: iPad 10.2, Samsung A70
- OS: iOS 14.6, Android 11

### Code snippet

```js
import {Button, Color, Composite, LayoutData, contentView} from 'tabris';

class Screen extends Composite {

constructor(background) {
super({
layoutData: LayoutData.stretch,
background
});
this.append(
new Button({
left: 16,
right: 16,
centerY: 0,
text: 'Click'
}).onSelect(async () => {
await new Promise(resolve => setTimeout(resolve, 4000)); // without this line it works fine
new Screen(Color.red).appendTo(contentView);
})
);
}

}

new Screen(Color.yellow).appendTo(contentView);
```