ExtJS sets the default value of the cascade menu _ extjs

ExtJS is very convenient to assign values on such pages. It can be seen in the 1.2.1 code in the body that a line of code can be done, but this is for common controls, such as text boxes. It's not that simple for ComboBox... Preface
ExtJS is very convenient to assign values on such pages. It can be seen in the 1.2.1 code in the body that a line of code can be done, but this is for common controls, such as text boxes. It's not that simple for ComboBox...

Version
Ext JS Library 3.0.0

Body
I. Problems
1.1
　　　　　　
Code 1.2
1.2.1 front-end code

The Code is as follows: