Although it happens on a number of screens, the application is large and complex, and I haven't yet been able to isolate a small test case. Will post if I do...
This may be unrelated, but I also get a number of exceptions from the following line in Ext.layout.component.Component.beginLayoutCycle:
The problem is that me.lastComponentSize is undefined the first time this method is called for the component (firstCycle == true). I haven't really investigated to see how this property is supposed to be "bootstrapped", but apparently, it doesn't happen the way it should in some cases...
ownerContext.setWidth(me.lastComponentSize.width, false, true);