title: WPF design view error
body: <p>I´m new at this job and when i opened a project i get this error and  i´m not able to figure out what´s going on.
  Any help?</p>
  
  <p>SO: WinXP Sp3, Visual Studio 2010 Express</p>
  
  <p>Thanks in advance.</p>
  
  <blockquote>
    <p>Exception not controlled.
    Details:
    System.Reflection.TargetInvocationException
    Se produjo una excepción en el destino de la invocación.
       en System.RuntimeMethodHandle._InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, SignatureStruct&amp; sig, MethodAttributes methodAttributes, RuntimeType typeOwner)
       en System.RuntimeMethodHandle.InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeType typeOwner)
       en System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
       en System.Delegate.DynamicInvokeImpl(Object[] args)
       en System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
       en MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)</p>
    
    <p>System.NullReferenceException
    Referencia a objeto no establecida como instancia de un objeto.
       en ControlPesajeLavanderia.ItemParte.NuevoItemParte_IsVisibleChanged(Object sender, DependencyPropertyChangedEventArgs e) en C:\Documents and Settings\golaizola\Mis documentos\Visual Studio 2010\Projects\ControlPesajeLavanderia\ControlPesajeLavanderia\usercontrols\ItemParte.xaml.vb:línea 186
       en System.Windows.UIElement.RaiseDependencyPropertyChanged(EventPrivateKey key, DependencyPropertyChangedEventArgs args)
       en System.Windows.UIElement.OnIsVisibleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
       en System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       en System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       en System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       en System.Windows.UIElement.UpdateIsVisibleCache()
       en System.Windows.UIElement.InvalidateForceInheritPropertyOnChildren(Visual v, DependencyProperty property)
       en System.Windows.UIElement.OnIsVisibleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
       en System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       en System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       en System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       en System.Windows.UIElement.UpdateIsVisibleCache()
       en System.Windows.UIElement.InvalidateForceInheritPropertyOnChildren(Visual v, DependencyProperty property)
       en System.Windows.UIElement.OnIsVisibleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
       en System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       en System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       en System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       en System.Windows.UIElement.UpdateIsVisibleCache()
       en System.Windows.UIElement.InvalidateForceInheritPropertyOnChildren(Visual v, DependencyProperty property)
       en System.Windows.UIElement.OnIsVisibleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
       en System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       en System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       en System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       en System.Windows.UIElement.UpdateIsVisibleCache()
       en System.Windows.UIElement.SynchronizeForceInheritProperties(UIElement uiElement, ContentElement contentElement, UIElement3D uiElement3D, DependencyObject parent)
       en System.Windows.UIElement.OnVisualParentChanged(DependencyObject oldParent)
       en System.Windows.FrameworkElement.OnVisualParentChanged(DependencyObject oldParent)
       en System.Windows.Media.Visual.FireOnVisualParentChanged(DependencyObject oldParent)
       en System.Windows.Media.Visual.AddVisualChild(Visual child)
       en System.Windows.FrameworkElement.set_TemplateChild(UIElement value)
       en System.Windows.Controls.ContentPresenter.UseContentTemplate.BuildVisualTree(FrameworkElement container)
       en System.Windows.StyleHelper.ApplyTemplateContent(UncommonField<code>1 dataField, DependencyObject container, FrameworkElementFactory templateRoot, Int32 lastChildIndex, HybridDictionary childIndexFromChildID, FrameworkTemplate frameworkTemplate)
       en System.Windows.FrameworkTemplate.ApplyTemplateContent(UncommonField</code>1 templateDataField, FrameworkElement container)
       en System.Windows.FrameworkElement.ApplyTemplate()
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Controls.Border.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Controls.Control.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Controls.Border.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en MS.Internal.Designer.ZoomableViewPresenter.DesignerBackground.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Controls.Decorator.MeasureOverride(Size constraint)
       en Microsoft.Windows.Design.Interaction.DesignerView.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en MS.Internal.Designer.Viewport.MeasureOverride(Size availableSize)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en MS.Internal.Helper.MeasureElementWithSingleChild(UIElement element, Size constraint)
       en System.Windows.Controls.ScrollContentPresenter.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Controls.Grid.MeasureCell(Int32 cell, Boolean forceInfinityV)
       en System.Windows.Controls.Grid.MeasureCellsGroup(Int32 cellsHead, Size referenceSize, Boolean ignoreDesiredSizeU, Boolean forceInfinityV)
       en System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Controls.ScrollViewer.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Controls.Grid.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en MS.Internal.Helper.MeasureElementWithSingleChild(UIElement element, Size constraint)
       en System.Windows.Controls.ContentPresenter.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Controls.Control.MeasureOverride(Size constraint)
       en System.Windows.FrameworkElement.MeasureCore(Size availableSize)
       en System.Windows.UIElement.Measure(Size availableSize)
       en System.Windows.Interop.HwndSource.SetLayoutSize()
       en System.Windows.Interop.HwndSource.set_RootVisualInternal(Visual value)
       en System.Windows.Interop.HwndSource.set_RootVisual(Visual value)
       en MS.Internal.DeferredHwndSource.ProcessQueue(Object sender, EventArgs e)</p>
  </blockquote>
  
  <p>UPDATE:</p>
  
  <p>I created a new form and copy all xaml until i get the error. The error happens when i included my own usercontrol in this line: 
  </p>
  
  <p>and the error says:
  "Object reference not set to an instance of an object."</p>
  
  <p>And I don´t know why...</p>