Sitecore 7:Publishing fails on Custom Save Handler - events

Question

I am trying to validate two date fields for start date to be before end date on item:saving event and below is the code for the same.
if (!string.IsNullOrEmpty(eventTemplate) && !string.IsNullOrEmpty(eventdate) && !string.IsNullOrEmpty(eventenddate))
{
//Validate ifitem is based on the event template then only doing the check
var validitem = (from temp in item.Template.BaseTemplates
where temp.ID == new Sitecore.Data.ID(eventTemplate)
select temp).FirstOrDefault();
if (validitem != null)
{
string firstFieldName = eventdate;
string lastFieldName = eventenddate;
Sitecore.Data.Fields.DateField first = item.Fields[firstFieldName];
Sitecore.Data.Fields.DateField last = item.Fields[lastFieldName];
if (last != null && last.DateTime != DateTime.MinValue && DateTime.Compare(first.DateTime, last.DateTime) > 0)
{
string firstTitle = Sitecore.StringUtil.GetString(first.InnerField.Title, first.InnerField.Name);
string lastTitle = Sitecore.StringUtil.GetString(last.InnerField.Title, last.InnerField.Name);
if (!string.IsNullOrEmpty(firstTitle) && !string.IsNullOrEmpty(lastTitle))
{
Sitecore.Web.UI.Sheer.SheerResponse.Alert(String.Format("Item not saved as {0} should be before or equal to {1}.", firstTitle, lastTitle));
}
eventArgs.Result.Cancel = true;
}
}
}
The above code works fine.However on publishing the site I get the below issue:
Job started: Publish to 'web'|#Exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at Sitecore.Web.UI.Sheer.ClientPage..ctor()
at Sitecore.Context.get_ClientPage()
at custom.ui.extensions.**ValidateDate.OnItemSaving**(Object sender, EventArgs args) in c:\Instance\source\ui\adiweb.ui.extensions\Custom Validations\ValidateDate.cs:line 46
at Sitecore.Events.Event.EventSubscribers.RaiseEvent(String eventName, Object[] parameters, EventResult result)
at Sitecore.Events.Event.RaiseEvent(String eventName, Object[] parameters)
at Sitecore.Events.Event.DataEngine_ItemSaving(Object sender, ExecutingEventArgs`1 e)
at System.EventHandler`1.Invoke(Object sender, TEventArgs e)
at Sitecore.Data.Engines.EngineCommand`2.CanExecute()
at Sitecore.Data.Engines.EngineCommand`2.Execute()
at Sitecore.Data.Engines.DataEngine.SaveItem(Item item)
at Sitecore.Data.Managers.ItemProvider.SaveItem(Item item)
at Sitecore.Data.Items.ItemEditing.AcceptChanges(Boolean updateStatistics, Boolean silent)
at Sitecore.Data.Items.EditContext.Dispose()
at Sitecore.Publishing.PublishHelper.CopyToTarget(Item sourceVersion)
at Sitecore.Publishing.PublishHelper.PublishVersionToTarget(Item sourceVersion, Item targetItem, Boolean targetCreated)
at Sitecore.Publishing.Pipelines.PublishItem.PerformAction.ExecuteAction(PublishItemContext context)
at Sitecore.Publishing.Pipelines.PublishItem.PerformAction.Process(PublishItemContext context)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Publishing.Pipelines.PublishItem.PublishItemPipeline.Run(PublishItemContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.ProcessEntries(IEnumerable`1 entries, PublishContext context)
at Sitecore.Publishing.Pipelines.Publish.ProcessQueue.Process(PublishContext context)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Publishing.Pipelines.Publish.PublishPipeline.Run(PublishContext context)
at Sitecore.Publishing.Publisher.Publish()
--- End of inner exception stack trace ---
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Jobs.Job.ThreadEntry(Object state)
Avy idea on why i get this issue.
Here error points to the line:
Sitecore.Web.UI.Sheer.SheerResponse.Alert(String.Format("Item not saved as {0} should be before or equal to {1}.", firstTitle, lastTitle));
IS use of Sitecore.Web.UI.Sheer.SheerResponse.Alert is invalid but i see many such implementations.

Answer 1

You're making a classical mistake here. The item: events such as item:saved and item:saving will be fired, when publishing. Basically because the act of publishing an item, means reading the item, and saving it onto the "web" database under normal circumstances.
The quickest fix you can implement, is to make sure you're dealing with item:saving on an item in the database you expect, probably "master". Something like:
if (validitem != null && validItem.Database.Name == "master")
A more long term fix, would be to hook your code into the "saveUI" pipeline instead - it triggers on what you expect here; when an item is being saved in the UI.

Answer 2

Since the item:saving event also runs during publishing, you should add a null check on Sitecore.Context.ClientPage to the start of your event handler, if it's null, return immediately. That is likely the source of the NullReferenceException.
if (Sitecore.Context.ClientPage == null) return;