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/HeaderButton.tsx

@@ -40,7 +40,7 @@ interface IProps {
 
 // TODO: replace this, the composer buttons and the right panel buttons with a unified representation
 export default class HeaderButton extends React.Component<IProps> {
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const { isHighlighted, isUnread = false, onClick, name, title, ...props } = this.props;
 
         const classes = classNames({

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()}

src/components/views/right_panel/TimelineCard.tsx

@@ -198,7 +198,7 @@ export default class TimelineCard extends React.Component<IProps, IState> {
         );
     };
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const highlightedEventId = this.state.isInitialEventHighlighted ? this.state.initialEventId : null;
 
         let jumpToBottom;

src/components/views/right_panel/VerificationPanel.tsx

@@ -332,7 +332,7 @@ export default class VerificationPanel extends React.PureComponent<IProps, IStat
         );
     }
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const { member, phase, request } = this.props;
 
         const displayName = (member as User).displayName || (member as RoomMember).name || member.userId;