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