title: Why do I get "TypeError: item.onAdded is not a function" when adding component to container?
body: <p>I am trying to create a framework UI that creates container contents based on configs, and I get an error "TypeError: item.onAdded is not a function".</p>
  
  <p>This does not happen in my smaller test app, but it does happen in our actual app. Can't figure out why.</p>
  
  <p>Here is some partial code.</p>
  
  <pre><code>Ext.define('atlas.screen.Viewport', {
      extend: 'Ext.container.Viewport',
      alias: 'widget.atlas.Viewport',
  
      autoScroll: true,
      layout: 'fit',
  
      requires: [
          'base.framework.MainAppView',
          'base.framework.MainAppNavigation'
      ],
  
      items: [{
          // MainAppView requires providing config with items to populate the north,
          // west, and center regions.
          xtype: 'mainAppView',
          westItems: [{
              xtype: 'mainAppNavigation'
          }]
       }]
  });
  
  Ext.define('base.framework.MainAppView', {
      extend: 'Ext.container.Container',
      alias: 'widget.mainAppView',
  
      requires: ['base.util.CommonBaseUtil'],
  
      autoScroll: true,
      layout: 'border',
  
      westItems: null,
  
      initComponent: function() {
          var me = this;
  
          Ext.applyIf(me, {
              items: [{
                  xtype: 'container',
                  itemId: 'appWestContainer',
                  region: 'west',
                  width: 85,
                  items: me.westItems,
                  hidden: true,
                  listeners: {
                      afterrender: CommonBaseUtil.showHide
                  }
              }]
          });
          me.callParent(arguments);
      },
  
      applyWestItems: function(westItems) {
          this.down('#appWestContainer').add(westItems);
      }
  });
  
  Ext.define('base.framework.MainAppNavigation', {
      extends: 'Ext.container.Container',
  
      alias: 'widget.mainAppNavigation',
  
      initComponent: function() {
          var me = this;
  
          Ext.applyIf(me, {
              items: [{
                  xtype: 'button',
                  itemId: 'myButton',
                  text: 'test'
              }]
          });
          me.callParent(arguments);
      }
  });
  </code></pre>