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/structures/NonUrgentToastContainer.tsx

@@ -45,7 +45,7 @@ export default class NonUrgentToastContainer extends React.PureComponent<IProps,
         this.setState({ toasts: NonUrgentToastStore.instance.components });
     };
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const toasts = this.state.toasts.map((t, i) => {
             return (
                 <div className="mx_NonUrgentToastContainer_toast" key={`toast-${i}`}>