I’d generally expect to have a document to be previewed before opening that window but I hear you!
I can help you with creating a keyboard shortcut though. Select “Tools > Keyboard Shortcuts > Edit” from the main menu, add a new shortcut and replace the main body with this:
#@begin-shortcut "default" [key="Ctrl+Alt+P", type="command"]
This will let you open the classic preview window with Ctrl+Alt+P