JavaScript Null Property Explainer

JavaScript's complaining it can't read a property of null or undefined and the stack trace tells you nothing useful. Paste the error and the code, and this explains in plain English what's null and why JavaScript is upset at that exact point, then shows the fix, telling you first whether it's a timing issue, a missing element, or bad data. For the error you've been Googling for half an hour.

I'm getting this JavaScript error: {ERROR_MESSAGE}. Code where it happens: {CODE_SNIPPET}. Explain in plain English what null or undefined means here, why JavaScript is complaining about it at this point in the code, and show me the fix. If the fix depends on whether this is a timing issue, a missing element, or bad data, explain which it is first.