Hi everybody! I suddenly started getting an error while publishing the page "Error: Object reference not set to an instance of an object."

Stack Trace:

>  [NullReferenceException: Object reference not set to an instance of
> an object.]   
> uComponents.DataTypes.MultiPickerRelations.<>c__DisplayClass1.<AfterSave>b__0(Property
> property) +50    System.Linq.WhereListIterator`1.MoveNext() +80   
> uComponents.DataTypes.MultiPickerRelations.MultiPickerRelationsEventHandler.AfterSave(Content
> sender, SaveEventArgs e) +269   
> umbraco.cms.businesslogic.web.SaveEventHandler.Invoke(Document sender,
> SaveEventArgs e) +0   
> umbraco.cms.businesslogic.web.Document.FireAfterSave(SaveEventArgs e)
> +89    umbraco.cms.businesslogic.web.Document.SaveAndPublishWithResult(User
> u) +546    umbraco.cms.presentation.editContent.Publish(Object sender,
> EventArgs e) +84   
> umbraco.controls.ContentControl.DoSaveAndPublish(Object sender,
> ImageClickEventArgs e) +108   
> System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e)
> +140    System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String
> eventArgument) +185    System.Web.UI.Page.ProcessRequestMain(Boolean
> includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
> +5027

What could be the reason? I can`t understand why it appeared and how to fix it