Conform more of the code base to strict null checking (#10147)

* Conform more of the code base to strict null checking

* More strict fixes

* More strict work

* Fix missing optional type

* Iterate

src/components/views/spaces/SpaceTreeLevel.tsx

@@ -299,7 +299,7 @@ export class SpaceItem extends React.PureComponent<IItemProps, IItemState> {
         }
     };
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         // eslint-disable-next-line @typescript-eslint/no-unused-vars
         const {
             space,