AS3 stops working if text button is modified

Question

I have a perfectly working text based button positioned inside a movie clip and being called from the main time line.
Then I decided to add a color changing effect on the up state of the button. Originally button had a static text for the Up, Dow, Over, Hit frames. As soon as I modified the text into a mc I am getting an error code:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at acolyte24A_fla::mainsite_mc_2/frame30()[acolyte24A_fla.mainsite_mc_2::frame30:4]
Here is line 4 from frame 30:
HowTo_maininfo_mc.HowTo_jtv4u_link_btn.addEventListener(MouseEvent.CLICK, onClick_openJTV);
"HowTo_jtv4u_link_btn" is the button in question. It was working right before I modified the Up state into an MC.
What is the proper steps to modify a button in this way?

Answer 1

Thank You.
I obviously had and still have some problems with tutorials from developphp, but 90% of my site is based on his staff. I always attributed the problems to my minimal level of expertise rather than his shortcomings. Would you be able to recommend a better, more reliable source of tutorials I can rely on?
I have added a line of code right before the line as you instructed.
So here is the line of code in question before I add the text line to an image based button:
HowTo_maininfo_mc.HowTo_jtv4u_link_btn.addEventListener(MouseEvent.CLICK, onClick_openJTV);
function onClick_openJTV(event:MouseEvent) :void {
navigateToURL(new URLRequest("http://www.ubloom.com/index.php?option=com_content&task=view&id=718&It emid=152"));
here how it looks when I add trace function:
trace(HowTo_maininfo_mc);
trace(HowTo_maininfo_mc.HowTo_jtv4u_link_btn);
HowTo_maininfo_mc.HowTo_jtv4u_link_btn.addEventListener(MouseEvent.CLICK, onClick_openJTV);
function onClick_openJTV(event:MouseEvent) :void {
navigateToURL(new URLRequest("http://www.ubloom.com/index.php?option=com_content&task=view&id=718&It emid=152"));
This is what I have in the OUTPUT panel:
[object HowTo_mainInfo_221]
[object SimpleButton]
[object HowTo_mainInfo_221]
[object SimpleButton]
The button is working and opens a site as intended.
Now I add a line of text to the button as my design intention is for user to see the link being written out and click on it. ( I do not want to do it as dynamic text because I want it to have Over and Out states where link lights up and in Up state it changes colors on the screen)
So after adding a line of text
[object HowTo_mainInfo_221]
null
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at acolyte24A_fla::mainsite_mc_2/frame30()[acolyte24A_fla.mainsite_mc_2::frame30:6]
I confirm I left the working button intact, all I did is added a text based mc to it.
Here is the line 6 from frame 30:
HowTo_maininfo_mc.HowTo_jtv4u_link_btn.addEventListener(MouseEvent.CLICK, onClick_openJTV);
How can it be a null object reference if it was just working before I added text, I did no even remove the drawing object from the button, just added the text?