numbers freezes upon opening

When I open the Numbers app, it freezes for a couple of minutes. I get a spinning wheel and a window that shows my documents folder, but I cannot select anything in that window. I can switch to other apps, but the keyboard does not work while Numbers is frozen. Sometimes Numbers eventually unfreezes and other times it shuts down and shows me an error report. When it's frozen, I notice that another program called com.apple.appkit.xpc.openAndSavePanel is running.

I'm using Yosemite, OS 10.10.4 on a new MacBook Pro.