我已经将Extjs 4.2升级到5.1.1，现在combobox getting error无法读取属性apply.。但在Extjs 4.2中，它工作得非常好

未捕获类型错误：无法读取未定义的ext all rtl debug.js？\u dc=1434006887415:122446Ext.define.getDisplayValue ext all rtl debug.js？\u dc=1434006887415:122446Ext.define.updateValue ext all rtl debug.js？\u dc=1434006887415:122405Ext.define.onValueCollectionEndUpdate ext all rtl debug.js？\u dc=143400887415:122187ext all rtl debug.js？\u dc=143400687415:11803doFireEvent ext all rtl debug.js？\u dc=1434006887415:12332fireEventArgs ext all rtl debug.js？\u dc=1434006887415:44074Ext.define.endUpdate ext all rtl debug.js？\u dc=143400887415:42996Ext.define.doSetValueext all rtl debug.js？_dc=1434006887415:122369Ext.define.setValue ext all rtl debug.js？_dc=1434006887415:122270Ext.define.setValueOnData ext all rtl debug.js？_dc=1434006887415:121833Ext.define.bindStore ext all rtl debug.js？_dc=1434006887415;:121774;.define.setStore ext all rtl debug.js？(dc=143400887415:78436（匿名函数）ext all rtl debug.js？_dc=1434006887415:6243Ext.define.bindStore ext all rtl debug.js？_dc=1434006887415:121769Ext.define.setStore ext all rtl debug.js？dc=1434006887415; ext all rtl debug.js？dc=1434006887415; 6645Ext.Base.Base.addMembers.initConfig ext all rtl debug.js？dc=143400887415:7424Ext.define.constructorext all rtl debug.js？_dc=1434006887415:39045构造函数ext all rtl debug.js？_dc=1434006887415:7660Ext.ClassManager.ext.apply.widget ext all rtl debug.js？_dc=1434006887415:8934Ext.define.create ext all rtl debug.js？_dc=1434006887415; 14599Ext.define.lookupComponent ext all rtl debug.js？_dc=1434006887415;.define.privates.76363;ext all rtl debug.js？_dc=1434006887415:76752Ext.define.add ext all rtl debug.js？_dc=1434006887415:75977Ext.define.initItems ext all rtl debug.js？_dc=1434006887415:76301Ext.Base.Base.addMembers.callParent ext all rtl debug.js？_dc=143400887415:7381;.define.initItems ext all rtl debug.js？_dc=143400887415:887415:88789Ext.Base.Base.addMembers.callParentext all rtl debug.js？_dc=1434006887415:7381Ext.define.initItems ext all rtl debug.js？_dc=1434006887415:119267Ext.define.initComponent ext all rtl debug.js？_dc=1434006887415:76269Ext.Base.Base.addMembers.callParent ext all rtl debug.js？_dc=1434006887415;:7381Ext.define.initComponentext all rtl debug.js？_dc=1434006887415:88783Ext.Base.Base.addMembers.callParent ext all rtl debug.js？_dc=1434006887415:7381Ext.define.initComponent ext all rtl debug.js？_dc=1434006887415:39089构造函数ext all rtl debug.js？_dc=1434006887415;.7660Ext.ClassManager.ext.apply.widgetext all rtl debug.js？_dc=1434006887415:8934Ext.define.create ext all rtl debug.js？_dc=1434006887415:14599Ext.define.lookupComponent ext all rtl debug.js？_dc=1434006887415:76363Ext.define.privates.prepareItems ext all rtl debug.js？_dc=1434006887415;=76752Ext all rtl debug.define.js？添加ext all rtl debug.js？_dc=143400887415:75977Ext.define.initItemsext all rtl debug.js？_dc=1434006887415:76301Ext.define.initComponent ext all rtl debug.js？_dc=1434006887415:76269Ext.Base.Base.addMembers.callParent ext all rtl debug.js？_dc=1434006887415:7381Ext.define.initComponent ext all rtl debug.js？_dc=1434006887415;=118849Ext.define.constructor ext all rtl debug.js？_dc=143400887415:89;ext all rtl debug.js？_dc=1434006887415:7660Ext.ClassManager.ext.apply.widget ext all rtl debug.js？_dc=1434006887415:8934Ext.define.create ext all rtl debug.js？_dc=1434006887415:14599Ext.define.lookupComponent ext all rtl debug.js？_dc=143400887415:76363Ext.define.privates.prepareItems ext all rtl debug.js？_dc=14340068877415:76752;ext all rtl debug.js？_dc=1434006887415:75977Ext.define.initItems ext all rtl debug.js？_dc=1434006887415:76301Ext.Base.Base.addMembers.callParent ext all rtl debug.js？_dc=1434006887415:7381Ext.define.initItems ext all rtl debug.js？_dc=1434006887415;:887415:88789Ext.Base.Base.addMembers.callParentext all rtl debug.js？_dc=1434006887415:7381Ext.define.initItems ext all rtl debug.js？_dc=1434006887415:119267Ext.define.initComponent ext all rtl debug.js？_dc=1434006887415:76269Ext.Base.Base.addMembers.callParent ext all rtl debug.js？_dc=1434006887415;:7381Ext.define.initComponentext all rtl debug.js？\u dc=1434006887415:88783Ext.Base.Base.addMembers.callParent ext all rtl debug.js？\u dc=1434006887415:7381Ext.define.initComponent ext all rtl debug.js？\u dc=143400887415:39089构造函数ext all rtl DEBUBUG.js？\u dc=143400887415:7660（匿名函数）VM2495:3Ext.ClassManager.Ext.apply.create Ext all rtl debug.js？\u dc=143400687415:8897Ext.ClassManager.Ext.apply.instantiatebylias Ext all rtl debug.js？\u dc=143400687415:8721Ext.Function.ExtFunction.alias Ext all rtl debug.js？\u dc=143400887415:4450Ext.define.OnSubMenuItemClick MenuDataView.js？\u dc=143400888341:51fireext all rtl debug.js？\u dc=143400687415:11803Ext.define.dispatch ext all rtl debug.js？\u dc=1434006887415:33357Ext.Base.Base.addMembers.callParent ext all rtl debug.js？\u dc=1434006887415:7381Ext.define.dispatch ext all rtl debug.js？\u dc=143400887415:33539prototype.dofirevent ext all rtl debug.js？\u dc=143400887415:33465firevents argsext all rtl debug.js？\u dc=1434006887415:12300fireEvent ext all rtl debug.js？\u dc=1434006887415:12274Ext.define.processUIEvent ext all rtl debug.js？\u dc=1434006887415:95171Ext.define.handleEvent ext all rtl debug.js？\u dc=1434006887415:11803Ext.define.fireext all rtl debug.js？_dc=1434006887415:18548Ext.define.publish ext all rtl debug.js？_dc=1434006887415:18524Ext.define.doDelegatedEvent ext all rtl debug.js？_dc=1434006887415:18561Ext.Function.ExtFunction.bind.method

Ext.override(Ext.form.ComboBox,
                {
                    getDisplayValue: function () { 
                        //return this.lastSelectionText;
                        return this.selection!=null?this.selection.get('DisplayValue'):this.lastSelection;
                    }
                });

I got the answer..