@Tim H Unfortunately, the answer is, "It depends." Yes, it is "other UI", but if in your use case, you know that it's always already fully set up before your WebDialog is opened (say, in response to user action) then it's safe to do. You have to think through the interaction between the server and the client. The main problem people run into is trying to manipulate UI that has either hasn't been created yet, or hasn't been received by the browser yet. The big difference between desktop and web is that delay between when the server has processed its code and when the browser has received the result and fully processed and displayed it to the user. If you fall into that gap, you get in trouble.
Thanks, Tim. I get the general idea here. Since, in this case, the dialog would have been called via a button on a WebPage, then referencing that WebPage's properties would be okay in the WebDialog's Open event because the WebPage would have been created at that point.