Support null ranges in the demo debug bar

The debug bar assumes ranges are never null. This also causes problems
with the 'Get range' button as pressing it blurs the document. To avoid
this just enable range logging onSelect all the time as it's pretty
useful and not very expensive (unlike 'Dump model').

Change-Id: I0a23bb7fb25b4259ee6ecf4b1b118c2974ee0b74