fuzzer web ui: fail scrolling into view gracefully

lib/fuzzer/main.js

@@ -214,7 +214,7 @@
     // Empirically, Firefox needs this requestAnimationFrame in order for the scrollIntoView to work.
     requestAnimationFrame(function() {
       const slDom = document.getElementById("l" + sourceLocationIndex);
-      slDom.scrollIntoView({
+      if (slDom != null) slDom.scrollIntoView({
         behavior: "smooth",
         block: "center",
       });