ExtendScript: 'Undefined is not an object' error everywhere? - object

Question

So I just checked out some ExtendScript code I wrote a few weeks back to finish the After Effects script I was working on.
I tried to compile, and now I get these 'Undefined is not an object' errors at various points in my code.
I won't post the full script now. Don't think that'd help the situation. Just why are these errors happening at all anyway? I didn't have them when I wrote the script, and now that I open it back up they're there. I really don't get it.
Anyway, let's take a look at e.g. this part of my code:
var projectCompNames = new Array ();
var projectCompObjects = new Array ();
var scriptPath = ((new File($.fileName)).parent).toString();
var i;
// get a list of all compositions in the project and write them into an array
for (i = 1; i <= app.project.numItems; i++) {
if (app.project.item(i) instanceof CompItem) {
projectCompNames.push(app.project.item(i).name);
projectCompObjects.push(app.project.item(i));
}
}
So as the comment suggests, this gets all the compositions in the project and writes them into an array. Worked fine when I worked on the script, now it doesn't.
This part:
app.project.numItems
Apparently produces the error 'Undefined is not object.' because if I change the code to this:
for (i = 1; i <= 5; i++) {
if (app.project.item(i) instanceof CompItem) {
projectCompNames.push(app.project.item(i).name);
projectCompObjects.push(app.project.item(i));
}
}
The error is gone. In the first line. Same error in the second line. Then I can fix it by using some random integers for the if condition and the error continues to be there. In any line of my code. All over it. I'm quite desperate here. No idea where this is coming from. Is there something wrong with the compiler? Should I reinstall the ExtendScript Toolkit? I mean, this is really odd.
If anyone knows why this is happening, I'd much appreciate the help. If I don't figure this out, I'm going to have to start over again with the script. I can't possibly debug that error at like 50 different places in my code, especially because it's so random.
Edit: Because I started the ExtendScript Editor individually and not from AE I had to link the editor to AE myself. I just didn't do that. If you launch it from the AE UI it does that automatically.
This is what I'm talking about. It was the only reason the compiler was freaking out. Quite obvious when all After Effects specific variables and functions weren't loaded.