You're not getting that error on the demo page, are you? If so, it almost has to be a non-fatal error. Either that, or it's a major bug and should be reported. Somehow I tend to doubt that's what's going on here. If not -
Do you really mean syntacticly? Then no, it's correct as far as that goes.
Without hunting through the script to find out where you pulled that one line from, I would guess it's part of some kind of loop and that the HTML markup you've associated with the script creates a situation whereby literally:
document.getElementById(tabinterfaceid) is null
If you want more help:
Please post a link to a page on your site that contains the problematic code so we can check it out.
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