main.js: handle document.activeElement being null

this is technically possible if someone sticks rustdoc in
an iframe, i think?

Author: lolbinarycat

Diff for: src/librustdoc/html/static/js/main.js

@@ -538,8 +538,8 @@ function preLoadCss(cssUrl) {
             return;
         }
 
-        // @ts-expect-error
-        if (document.activeElement.tagName === "INPUT" &&
+        if (document.activeElement &&
+            document.activeElement.tagName === "INPUT" &&
             // @ts-expect-error
             document.activeElement.type !== "checkbox" &&
             // @ts-expect-error