Add sanity check (#115)

Author: wilsonge

src/js/tab/tab.js

@@ -385,6 +385,10 @@
     findAncestor(el, tagName) {
       let element = el;
       while (element.nodeName.toLowerCase() !== tagName) {
+        // Ensure we haven't reached the top of the dom tree
+        if (element.parentElement === null) {
+          return;
+        }
         element = element.parentElement;
       }
       return element;