fix #38891: phantom measures after delete all in continuous view

As I mention in the issue report, I don't fully understand what is going on, but I can see that we need to clear the bspTree and aren't.  So I decided to try passing in 0 for h and/or w in cases where there are no systems and/or measures.  It works in the tests I did, and still fixes the crashes that the code I am replacing was designed to fix (as far as I can tell).