Fix offsetHeight console error

This commit fixes the console error `there is an error in the
console "cannot read property offsetHeight of null"`

app/assets/javascripts/lib/utils/common_utils.js

@@ -123,7 +123,7 @@ export const handleLocationHash = () => {
   }
 
   if (isInIssuePage()) {
-    adjustment -= fixedIssuableTitle.offsetHeight;
+    adjustment -= fixedIssuableTitle?.offsetHeight;
   }
 
   if (isInMRPage()) {