Prevent invalid height value of MainContent when using the `fullHeigh…

…t` prop

Previously, `topPosition` could be `undefined`, causing the `height` to
be invalid because `calc()` contained the value `undefinedpx`.

This issue would occur on the page tree when clicking on a page and
then clicking the back button. The page tree's `MainContent` would
have a height of `0`.

.changeset/quiet-paws-rush.md

@@ -0,0 +1,7 @@
+---
+"@comet/admin": patch
+---
+
+Prevent the `MainContent` component from having an invalid height.
+
+This bug would cause the page tree to have no height when navigating into a page and then navigating back again.

packages/admin/admin/src/mui/MainContent.tsx

@@ -43,7 +43,7 @@ function Main({
     classes,
 }: MainContentProps & WithStyles<typeof styles>) {
     const mainRef = React.useRef<HTMLElement>(null);
-    const topPosition = fullHeight ? mainRef.current?.offsetTop : 0;
+    const topPosition = fullHeight && mainRef.current ? mainRef.current.offsetTop : 0;
 
     const rootClasses: string[] = [classes.root];
     if (disablePaddingTop) rootClasses.push(classes.disablePaddingTop);