How to use document.querySelector in a forge Custom UI App?

Forge Forge UI Kit and Custom UI
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1

I tried using document.querySelector in a forge App that is opening createIssueModal where I am trying to access the heading of the create Issue Modal. I get null always while the same querySelector is working fine in the browser console.

2

Custom UI apps work inside an iframe, so they don’t have access to the same document as the browser console.

3

Thanks @AaronCollier. That helps.