don't execute if kernel is not connected

IPython/html/static/notebook/js/codecell.js

@@ -278,6 +278,11 b' define(['
      * @method execute
      */
     CodeCell.prototype.execute = function () {
+        if (!this.kernel || !this.kernel.is_connected()) {
+            console.log("Can't execute, kernel is not connected.");
+            return;
+        }
+        
         this.output_area.clear_output();
         
         // Clear widget area