Error in WPF Visual Studio Designer

Question

I'm getting this weird error in the designer.
Visual Studio 2013 WPF - other views/windows are working fine. This is happening when I have design time data (if I remove the design time data it works fine).

System.NullReferenceException Object reference not set to an instance of an object.
at System.Windows.Controls.GridViewHeaderRowPresenter.OnLayoutUpdated(Object sender, EventArgs e)
at System.Windows.ContextLayoutManager.fireLayoutUpdateEvent()
at System.Windows.ContextLayoutManager.UpdateLayout()
at System.Windows.UIElement.UpdateLayout()
at System.Windows.Interop.HwndSource.SetLayoutSize()
at System.Windows.Interop.HwndSource.set_RootVisualInternal(Visual value)
at System.Windows.Interop.HwndSource.set_RootVisual(Visual value)
at Microsoft.Expression.DesignHost.Isolation.Remoting.RemoteUIElement.<>c__DisplayClass10.b__f()
at Microsoft.Expression.DesignHost.Isolation.Remoting.ThreadMarshaler.<>c__DisplayClass2a`1.b__29()
at Microsoft.Expression.DesignHost.Isolation.Remoting.STAMarshaler.Call.InvokeWorker()
at Microsoft.Expression.DesignHost.Isolation.Remoting.STAMarshaler.Call.Invoke(Boolean waitingInExternalCall)
at< Microsoft.Expression.DesignHost.Isolation.Remoting.STAMarshaler.InvokeCall(Call call)
at Microsoft.Expression.DesignHost.Isolation.Remoting.STAMarshaler.ProcessQueue(CallQueue queue)
at Microsoft.Expression.DesignHost.Isolation.Remoting.STAMarshaler.ProcessInboundAsyncQueue(Int32 identity)
at Microsoft.Expression.DesignHost.Isolation.Remoting.STAMarshaler.ProcessMessage(Int32 msg, IntPtr wParam, IntPtr lParam, Boolean elevatedQuery, Boolean& handled)
at Microsoft.Expression.DesignHost.Isolation.Remoting.STAMarshaler.OnWindowMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at Microsoft.Expression.DesignHost.Isolation.Remoting.MessageOnlyHwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
at System.Windows.Threading.Dispatcher.Run()
at System.Windows.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at System.Windows.Application.Run(Window window)
at Microsoft.Expression.DesignHost.Isolation.DesignerProcess.RunApplication()
at Microsoft.Expression.DesignHost.Isolation.DesignerProcess.<>c__DisplayClass2.b__0()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()

Image of error

Answer 1

This was caused by some converters not checking for null: I added in the check for value == null and all is fine again.

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)

    {
        if (value == null) { return "White"; }
        var valueAsString = (string)value;

        if ( String.IsNullOrWhiteSpace(valueAsString) )
        {
            return "White";
        }

        if (valueAsString.ToLower().Contains("Late".ToLower()))
        {
            return "Yellow";
        }
        return "White";         
    }

MSDN also has some pointers on design time errors here: http://msdn.microsoft.com/query/dev12.query?appId=Dev12IDEF1&l=EN-US&k=k(VS.XamlDesigner.ExceptionUI);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.0)&rd=true

Value Converters Your custom IValueConverter implementations should check for null and for the expected type in the first parameter of the Convert method. The following XAML shows a binding to Application.Current that fails at design time if the value converter is not implemented correctly.