Fix: ignore the initial empty ec-app

The initial CC HTML contains an empty ec-app element, and the script
sometimes tried to initialize the intersection observer with this empty
ec-app element.

Now if the ec-app element doesn't contain the |.scrollable-content| div
(which means it's empty) the intersection observer is not initialized.

This ec-app element is always replaced later on with another ec-app
element which does contain the |.scrollable-content| div.

Bug: #21
Change-Id: I2dc54d0edf4a010a3860ffa1bdb104c8303de911
diff --git a/src/content_scripts/console_inject.js b/src/content_scripts/console_inject.js
index ab0abca..8a1d373 100644
--- a/src/content_scripts/console_inject.js
+++ b/src/content_scripts/console_inject.js
@@ -239,14 +239,17 @@
     // Set up the intersectionObserver
     if (typeof intersectionObserver === 'undefined' && ('tagName' in node) &&
         node.tagName == 'EC-APP') {
-      intersectionOptions = {
-        root: node.querySelector('.scrollable-content'),
-        rootMargin: '0px',
-        threshold: 1.0,
-      };
+      var scrollableContent = node.querySelector('.scrollable-content');
+      if (scrollableContent !== null) {
+        intersectionOptions = {
+          root: scrollableContent,
+          rootMargin: '0px',
+          threshold: 1.0,
+        };
 
-      intersectionObserver =
-          new IntersectionObserver(intersectionCallback, intersectionOptions);
+        intersectionObserver =
+            new IntersectionObserver(intersectionCallback, intersectionOptions);
+      }
     }
 
     // Start the intersectionObserver for the "load more"/"load all" buttons