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/dialogs/ErrorDialog.tsx

@@ -52,7 +52,7 @@ export default class ErrorDialog extends React.Component<IProps, IState> {
         this.props.onFinished(true);
     };
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         return (
             <BaseDialog
                 className="mx_ErrorDialog"