dillon1000/react

Improve Flow coverage by reducing typecasts through `any`

There are many unnecessary typecasts through `any` which
break the Flow of types across the program.

It's more bulletproof to avoid lying to ourselves about types.

Also fixed sketchy null check where zero ID would be skipped:
```diff
-  } else if (selectedElementID) {
+  } else if (selectedElementID !== null) {
```