Prevent access to this.$sticky unless it is ready

This change adds checks to ensure that this.$sticky exists and it is visible
before trying to access its methods.

Further information at #368

lib/assets/javascripts/_modules/table-of-contents.js

@@ -10,7 +10,7 @@
     this.isMonitoring = false
   }
   StickyOverlapMonitors.prototype.run = function () {
-    var stickyIsVisible = this.$sticky.is(':visible')
+    var stickyIsVisible = this.$sticky && this.$sticky.is(':visible')
     if (stickyIsVisible && !this.isMonitoring) {
       document.addEventListener('focus', this.onFocus, true)
       this.isMonitoring = true
@@ -26,11 +26,13 @@
 
     if (!applicable) { return }
 
-    var stickyEdge = this.$sticky.get(0).getBoundingClientRect().bottom + this.offset
-    var diff = focused.getBoundingClientRect().top - stickyEdge
+    if (this.$sticky && this.$sticky.is(':visible') && this.$sticky.get(0)) {
+      var stickyEdge = this.$sticky.get(0).getBoundingClientRect().bottom + this.offset
+      var diff = focused.getBoundingClientRect().top - stickyEdge
 
-    if (diff < 0) {
-      $(window).scrollTop($(window).scrollTop() + diff)
+      if (diff < 0) {
+        $(window).scrollTop($(window).scrollTop() + diff)
+      }
     }
   }