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/FilePanel.tsx

@@ -223,7 +223,7 @@ class FilePanel extends React.Component<IProps, IState> {
         }
     }
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         if (MatrixClientPeg.get().isGuest()) {
             return (
                 <BaseCard className="mx_FilePanel mx_RoomView_messageListWrapper" onClose={this.props.onClose}>