Multiple refresh requests from Customizer preview cause Uncaught TypeError: this.targetWindow is not a function

[westonruter]

Add a button to the Customizer preview which sends two refresh messages back-to-back to the pane:

wp.customize.preview.send( 'refresh' );
wp.customize.preview.send( 'refresh' );

This will also trigger the error.

Uncaught TypeError: this.targetWindow is not a function

This error is occurring specifically in wp.customize.Messenger.prototype.receive() where it does

if ( ! this.targetWindow() )
        return;

The problem is that when a refresh message is received, the wp.customize.PreviewFrame.prototype.destroy() method will get called which does:

delete this.targetWindow;

And so for the second message received, the this.targetWindow variable will be undefined.

The simplest fix here is to just make sure that this.targetWindow is defined before checking its value.

This was found when testing #27355.

Note that this won't be relevant once the iframe is loaded with the natural URL (#30028).

[westonruter]

In 36583:

Customize: Prevent consecutive refresh requests from preview from causing JS error.

Fixes "Uncaught TypeError: this.targetWindow is not a function".

See #27355.
Fixes #35866.