Fix strictNullChecks errors in AccessibilityManager

[Tyriar]

Part of #1319

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@mofux @parisk this is what strictNullChecks looks like, the changes were necessary to clear all these errors:

src/AccessibilityManager.ts(97,5): error TS2322: Type 'null' is not assignable to type 'IDisposable[]'.
src/AccessibilityManager.ts(98,5): error TS2322: Type 'null' is not assignable to type 'HTMLElement'.
src/AccessibilityManager.ts(99,5): error TS2322: Type 'null' is not assignable to type 'HTMLElement'.
src/AccessibilityManager.ts(100,5): error TS2322: Type 'null' is not assignable to type 'HTMLElement'.
src/AccessibilityManager.ts(101,5): error TS2322: Type 'null' is not assignable to type 'HTMLElement'.
src/AccessibilityManager.ts(102,5): error TS2322: Type 'null' is not assignable to type 'HTMLElement[]'.
src/AccessibilityManager.ts(127,7): error TS2322: Type 'HTMLElement | undefined' is not assignable to type 'HTMLElement'.
  Type 'undefined' is not assignable to type 'HTMLElement'.
src/AccessibilityManager.ts(130,7): error TS2322: Type 'HTMLElement | undefined' is not assignable to type 'HTMLElement'.
  Type 'undefined' is not assignable to type 'HTMLElement'.
src/AccessibilityManager.ts(176,38): error TS2345: Argument of type 'HTMLElement | undefined' is not assignable to parameter oftype 'Node'.
  Type 'undefined' is not assignable to type 'Node'.
src/AccessibilityManager.ts(220,13): error TS2531: Object is possibly 'null'.

Trades a little bit of flexibility for more compile errors, the most annoying thing to deal with was .pop() on HTMLElement[] would return HTMLElement | undefined. Anyone against using this?

[parisk]

I think the trade-off is totally OK.

LGTM 🍣