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/right_panel/HeaderButtons.tsx

@@ -97,7 +97,7 @@ export default abstract class HeaderButtons<P = {}> extends React.Component<IPro
     // XXX: Make renderButtons a prop
     public abstract renderButtons(): JSX.Element;
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         return (
             <div className="mx_HeaderButtons" role="tablist">
                 {this.renderButtons()}