Happens both in classic and in modern.

modern
Code:
w = new Ext.Widget();
w.destroy();
// wait for destroy setting w.config to null
Ext.on('idle', function() {w.getConfig('viewModel', true)})
classic
Code:
c = new Ext.Component();
c.destroy();
// wait for destroy setting c.config to null
Ext.on('idle', function() {c.getConfig('viewModel', true)})
Causes:
Code:
Uncaught TypeError: Cannot read property 'viewModel' of null
This causes problems in calls to _isHierarchicallyHidden_ or more precise _initInheritedState_,