Fix error on 'validationMessage' custom binding.

When the 'validationMessage' custom binding is used in an element, and
that element is later removed from the DOM, if the validation is
triggered an error is thrown.

This PR fixes that problem by checking on whether the element has a
parent node before updating it, and by disposing the subscription that
updates the element when the element is disposed.

spec/validation-element-spec.js

@@ -100,5 +100,16 @@ describe('Validation message element', function () {
       expect(messageElement).toHaveClass('validation-message');
       expect(messageElement).toHaveClass('validation-fixed');
     });
+
+    it('does not throw an error when the element is removed from the dom and the validation is triggered', function () {
+      function removeValidationMessageElementAndValidate() {
+        $('#messageElement').remove();
+
+        $('#firstName').val('').trigger('change');
+        $('#firstName').val('Chuck').trigger('change');
+      }
+
+      expect(removeValidationMessageElementAndValidate).not.toThrow();
+    });
   });
 });

src/ko-validation.js

@@ -140,11 +140,13 @@ ko.validation.registerValidator = function (name, validatorFactory) {
       };
     });
     ko.bindingHandlers.text.update(element, observable.validationMessage);
-    ko.bindingHandlers.css.update(element.parentNode, function () {
-      return {
-        'error': observable.validationState() === ko.validation.validationStates.INVALID
-      };
-    });
+    if (element.parentNode) {
+      ko.bindingHandlers.css.update(element.parentNode, function () {
+        return {
+          'error': observable.validationState() === ko.validation.validationStates.INVALID
+        };
+      });
+    }
   }
 
   function initValidationFor(inputElement, observable) {
@@ -184,8 +186,9 @@ ko.validation.registerValidator = function (name, validatorFactory) {
 
   ko.bindingHandlers.validationMessage = {
     init: function (element, valueAccessor) {
-      var observable = valueAccessor();
+      var observable, subscription;
 
+      observable = valueAccessor();
       if (!ko.validation.utils.hasValidators(observable)) {
         throw new Error("'validationMessage' should be used with an observable that has validation");
       }
@@ -194,9 +197,12 @@ ko.validation.registerValidator = function (name, validatorFactory) {
       ko.bindingHandlers.css.update(element, function () {
         return { 'validation-message': true };
       });
-      observable.validationState.subscribe(function () {
+      subscription = observable.validationState.subscribe(function () {
         updateValidationMessage(element, observable);
       });
+      ko.utils.domNodeDisposal.addDisposeCallback(element, function () {
+        subscription.dispose();
+      });
     }
   };