document.getElementById('txtValue').focus(); Fails with an error on MODAL popups

I have the JS:

Code:

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document.getElementById('txtValue').focus();

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This works fine is I call the page directly, or if the page is shown in a popup window that is NON modal.

When I show this using the JS:

Code:

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window.showModalDialog

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Then I get the following attached error.

Woka

Here's the error when not running the IDe in debug mode.

Woka