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/messages/MessageEvent.tsx

@@ -149,7 +149,7 @@ export default class MessageEvent extends React.Component<IProps> implements IMe
         this.forceUpdate();
     };
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const content = this.props.mxEvent.getContent();
         const type = this.props.mxEvent.getType();
         const msgtype = content.msgtype;