Help with Errors please!

Question

Hi, I'm building a Flash website.
I've used frames with names to contain the different 'pages'
of the site. On the frame called 'SVHS1' I've built an XML mp3
player with buttons and a volume slider.
I'm having a couple of problems with this section of the
site.
Firstly: when I 'test movie' and navigate to the mp3 player
section it all works fine, the mp3s load correctly, the playback
controls work fine, and the volume slider does what it's supposed
to do. But if I've used the volume slider to adjust the volume (and
only then) and then navigate to the next frame 'SVHS2' (or any
frames that are later in the main timeline) I get multiple
occurrences of the following error in the output pane:
TypeError: Error #1009: Cannot access a property or method
of a null object reference.
At PSPWebsite1_fla::MainTimeline/adjustVolume()
I understand (from reading other posts/help etc.) in
principle what is causing this, but my knowledge of AS3 is too
limited to know exactly what is wrong with my script or how to fix
it.
Secondly: if I press the stop button on my mp3 player and
then navigate to another frame (any frame before or after
‘SVHS1’) the site works fine, and the mp3 player works
perfectly if I go to that frame again. But if I navigate to another
frame without first stopping the song it continues to play even
though I’m not on the mp3 player frame, and if I go back to
the mp3 player before the song has finished it starts playing
automatically from the beginning of the first song in the XML list
again and simply doubles up the sound, which is horrible. When this
happens the playback controls only affect the 2nd version
(understandably).
I think this is probably quite simple to fix, assuming that I
have to write some script that tells the mp3 player to stop playing
if/when the user navigates away from that frame...but I don’t
know how to do this.
Any help with either of the above would be much appreciated.
The attached script is for the mp3 player frame.

Answer 1

You can consider the trace function as being a "can you hear
me now" helper (instead of an annoying tv commercial gimmick). It
displays whatever it's told to in the same output window that you
see that error message come up in. There are two ways I'll usually
use it.
I'll often end up simply using a trace("here"); as a means of
seeing where a process is getting hung up. I gradually move that
line of code thru what I know to be the step by step sequence of
the code until "here" no longer shows up. It normally indicates
that the preceding line of code is where the ball got dropped.
In AS3, which is often a little more helpful when it trips on
something, such as the error message you received which tells you
there's a problem in the adjustVolume function, you can use it to
see what element in that function is either undefined or out of
scope.
Place... trace(vol); ... just after the first line of code in
that function to see if vol is a number. If it is, then you know
that line is okay and you can move on to check the next line. If it
isn't a number, then move the trace before that line and change it
to... trace(volume_mc.slider_mc.x); ... and see if it is a value.
If it isn't, then change the trace to check to see what
trace(volume_mc); results in.... so on and so on.
It's basically a process of working your way thru code to
narrow down what piece is the undefined/out-of-scope (or a syntax
error) element that is causing the problem.