Revert "`getActiveCodeEditor` should return undefined instead of null"
Revert "Make `getFocusedCodeEditor` return undefined instead of null"

This reverts commit db11576d.
This reverts commit 0fdba099.
This reverts commit b18ef0e7

related to #78168

Emitters should generally not be reassigned as their corresponding event would also be lost. Marking these as readonly just to enforce this

#70384

fixes #76419

Our incremental build missed these

`clear` disposes of all tracked disposables but does not mark the store itself as disposed. If you need a mutatable list of disposables, this allows you to always use `readonly`/`const` disposable stores that can be registered on the base class.

Part of #74250

- Extend `Disposable` in classes where it makes sense
- Use `DisposableStore` for lists of disposables
- Make `combinedDisposable` take arguments instead of an array (so that you can't pass in an array and then modify the array after the fact)

#70020

**Problem**
`ITheme.getColor` is one of the major users of the type `null` in our codebase. However this function is used in many places that expected an `undefined` type instead. With the strict null work, this means that we have to write code such as `theme.getColor(...) || undefined` in some cases or have to work with  `Color | null | undefined` types.

**Fix**
`undefined` is generally better supported by TypeScript and more natural in JavaScript. This change makes `ITheme.getColor` return `Color | undefined` instead of `Color | null`.

related to #67027