Extjs报错缺少对象。经常遇到这种错误，这个错误是因为什么？在线等待

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Ext.onReady(function(){
Ext.create('Ext.container.Viewport',{//这句报缺少对象。。。。求真相。。
layout: 'border',
renderTo:'port',
items: [{
region: 'west',
collapsible: true,
title: 'Navigation',
xtype: 'treepanel',
width: 200,
autoScroll: true,
split: true,
loader: new Ext.tree.TreeLoader(),
root: new Ext.tree.AsyncTreeNode({
expanded: true,
children: [{
text: 'Menu Option 1',
leaf: true
}, {
text: 'Menu Option 2',
leaf: true
}, {
text: 'Menu Option 3',
leaf: true
}]
}),
rootVisible: false,
listeners: {
click: function(n) {
Ext.Msg.alert('Navigation Tree Click', 'You clicked: "' + n.attributes.text + '"');
}
}
}, {
region: 'center',
html:'html'

}]
});
});