Allow null model value and ensure tinymce is instantiated

First fix just ensures that `undefined` doesn't get passed to `setContent` because I found that `$viewValue` can be null if using ngRequired directive.

Second fix uses an interval loop to keep checking for the tinymce editor object because otherwise `tinyInstance` can be undefined since `tinymce.init` is in a `setTimeout`.

Author: jrust

src/tinymce.js

@@ -60,12 +60,18 @@ angular.module('ui.tinymce', [])
         setTimeout(function () {
           tinymce.init(options);
         });
-        ngModel.$render = function(){
-          if (!tinyInstance) {
-            tinyInstance = tinymce.get(attrs.id);
+        
+        var interval = setInterval(function (){
+          tinyInstance = tinymce.get(attrs.id);
+
+          if (tinyInstance) {
+            clearInterval(interval);
+
+            ngModel.$render = function() {
+              tinyInstance.setContent(ngModel.$viewValue || '');
+            };
           }
-          tinyInstance.setContent(ngModel.$viewValue);
-        };
+        }, 0);
       }
     };
   }]);