Fix injected script's Shadow DOM parsing

Fixes the _exploreShadowDOM function defined in the
js/injected_script.js to check for root's children before recursively
exploring them.

common/js/injected_script.js

@@ -248,6 +248,8 @@ class InjectedScript {
       );
       result = result.concat(shadowRootResults);
     }
+
+    if (root.children === undefined) return result;
 
     for (let i = 0; i < root.children.length; i++) {
       const childElement = root.children[i];