TypeError: Error #1009: Cannot access a property or method of a null object reference

1

Hey guys,
I know this has been commented on before but I can’t for the life of me figure out what is going on in my code.

I get the error:
**
TypeError: Error #1009: Cannot access a property or method of a null object reference.**


Next1.addEventListener(MouseEvent.CLICK, Next1Handler); 
**Next1.addEventListener(MouseEvent.MOUSE_OVER, Next1Over); 
Next1.addEventListener(MouseEvent.MOUSE_OUT, Next1Out); **
function Next1Handler(event:MouseEvent):void { 
        returnPage = "screen1"; 
        if ((exitType == "ET0")||(voltType == "VT0")){ 
                Close1.visible = false; 
                Text1.visible = true; 
                Text1.htmlText = helpData.M_1; 
        } else { 
                if (exitType == "ET1") {gotoAndStop("screen2a");} 
                if (exitType == "ET2") {gotoAndStop("screen2b");} 
        } 
}

**function Next1Over(event:MouseEvent) :void 
{ 
        Next1.gotoAndPlay("over");      
} 
function Next1Out(event:MouseEvent) :void 
{ 
        Next1.gotoAndPlay("out");       
}**

From the bolded lines of code. I might just be looking past the obvious but I can’t see what is wrong with any of those lines.