Fix exception when drawAttention is called too early

// FREEBIE

js/chromium.js

@@ -135,18 +135,22 @@
             console.log('draw attention');
             if (chrome.app.window) {
                 var w = chrome.app.window.get(window_id);
+                if (w) {
                     w.clearAttention();
                     w.drawAttention();
                 }
+            }
         },
 
         clearAttention: function(window_id) {
             console.log('clear attention');
             if (chrome.app.window) {
                 var w = chrome.app.window.get(window_id);
+                if (w) {
                     w.clearAttention();
                 }
             }
+        }
 
     };