You tried to access an external html page, which can not be opened on the device for preview. open the page in dreamweaver to preview the page on the device. Real-Time Preview is a new feature in this version. The developers didn't offer an option to turn it off in Preferences. So, the best thing to do is to Right-Click on your document tab (or a file in your Files Panel) and select Open in Browser. Then, choose your preferred browser. As a reminder, don't use F12.
