A project without a time signature will freeze if you select a rest and then press ‘R’ approximately ten times.
To reproduce this issue:
I have recreated this issue several times. The first time I noticed it was when I added a new flow to a project and wanted to quickly create some empty bars, but I forgot to add the time signature first.
If there is no time signature, Dorico cannot create bars, since the bar has no defined length!
This makes no sense in Dorico: Rests aren’t “real” objects unless they’re forced. When you want to lengthen a flow that has no meter, use Shift-B to add beats such as +16q or something. Or just write notes and don’t worry about the rests or the length!
Dorico’s not freezing, but it’s unable to fit the by now very long bar into the width of the frame, so it can’t draw anything at all. Each time you hit R you’re doubling the length of the flow, so by the time you’ve done this 10 times, your open meter bar is 1024 quarter notes (crotchets) in length, which is definitely too much to fit on one system.