Conform more of the codebase to strictNullChecks (#10350

* Conform more of the codebase to `strictNullChecks`

* Iterate

* Generics ftw

* Iterate

src/components/structures/LeftPanel.tsx

@@ -271,6 +271,7 @@ export default class LeftPanel extends React.Component<IProps, IState> {
         // add appropriate sticky classes to wrapper so it has
         // the necessary top/bottom padding to put the sticky header in
         const listWrapper = list.parentElement; // .mx_LeftPanel_roomListWrapper
+        if (!listWrapper) return;
         if (lastTopHeader) {
             listWrapper.classList.add("mx_LeftPanel_roomListWrapper_stickyTop");
         } else {