Removing error when hovering over non-heap squares. Fixes pr11147

collects/plai/private/gc-gui.rkt

@@ -318,6 +318,7 @@
          (update-arrows '())]))
 
     (define/private (index->nexts index)
+      (if (< index (vector-length heap-vec))
           (let ([n (vector-ref heap-vec index)])
             (cond
               [(and (exact-integer? n)
@@ -327,7 +328,8 @@
               [(procedure? n)
                (map read-root (procedure-roots n))]
               [else
-           '()])))
+               '()]))
+          '()))
     
     (define/private (find-connections start)
       (let ([visited (make-hash)]