wpf - Style border and Title Bar cause NullReferenceException -

i want change form border , title bar , fount this solution. after add xaml (i using first solution in first answer) application running , can see style changed after few seconds can see exception in designer:

my application still run , show new style happening again , again, try remove , add changes several times error still jump after while. update

this code added xaml:

<window x:class="csharpwpf.mainwindow"              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"             title="mainwindow" >     <windowchrome.windowchrome>         <windowchrome captionheight="{binding actualheight,elementname=titlebar}"/>     </windowchrome.windowchrome>     <dockpanel lastchildfill="true">         <border background="lightblue" dockpanel.dock="top" height="25" x:name="titlebar">             <textblock text="{binding title, relativesource={relativesource findancestor,ancestortype=window},fallbackvalue=title}"                         margin="10,0,0,0"                        verticalalignment="center">                 <textblock.effect>                     <dropshadoweffect color="white" shadowdepth="3"/>                 </textblock.effect>             </textblock>         </border>         <border borderbrush="lightgray" borderthickness="1" padding="4">             <textblock text="window content"/>         </border>     </dockpanel> </window>