ActionScript 3.0 :: Capture Input Text From User - Buttons Generate Errors

I'm using AS3 in timeline. I have a three page movieclip (dynamic_mc). Each page is labeled p1..p3. On p1 I have an input text box named userName and button named next_1. On p2 I have a dynamic text box named greetings_1, an input text field named schoolName and two buttons (back1 and next2).On p3 I have another dynamic text box named greetings_2 and two buttons (back_2 and next_3).

On p1 user enters name, which is captured on p2. Once a click next button on p1, I get this error:
TypeError: Error #1009: Cannot access a property or method of a null object reference. at FlashCS4_Curriculum_fla::lesson9_pg6_dynamic_122/frame10() at flash.display::MovieClip/gotoAndStop() at FlashCS4_Curriculum_fla::lesson9_pg6_dynamic_122/goP2()
Why do I get an error when using buttons? Is there a better way to capture input text?

Here is the code by page.
//page 1 = p1stop();
var hisName:String;
next_1.addEventListener(MouseEvent.CLICK, goP2);function goP2(evt:MouseEvent): void{ hisName =userName.text; gotoAndStop("p2");
} // page 2 =p2 greetings_1.text= "Welcome :" +hisName;var school:String;
//next buttonnext_2.addEventListener(MouseEvent.CLICK, goP3);
[Code] .....