c# - WPF Expander ScrollViewer throwing exception when window is closed -

i'm showing expander in modal window. if don't toggle expander window close fine. once i've toggled , close window throws exception on invocation of

system.windows.media.animation.doubleanimationbase.getcurrentvalue(object defaultoriginvalue, object defaultdestinationvalue, animationclock animationclock) 

i have same control on window works fine.

i define control follows

 <expander template="{dynamicresource moreinformationexpander}">       <stackpanel>           <textblock text="{binding moreinformationtext}  />        </stackpanel>  </expander> 

the template expander:

<controltemplate x:key="moreinformationexpander" targettype="{x:type expander}">     <dockpanel horizontalalignment="stretch">         <togglebutton x:name="expanderbutton"                            dockpanel.dock="bottom"                           template="{staticresource moreinformationexpanderbutton}"                           ischecked="{binding path=isexpanded, relativesource={relativesource templatedparent}}"                           overridesdefaultstyle="true"                           horizontalalignment="left">         </togglebutton>         <scrollviewer x:name="expandercontentscrollview" dockpanel.dock="top"                           horizontalscrollbarvisibility="hidden"                           verticalscrollbarvisibility="hidden"                           horizontalalignment="stretch"                           horizontalcontentalignment="stretch"                           verticalcontentalignment="bottom"                           background="{dynamicresource windowbordergraybrush}"                           padding="8,4,0,0">             <scrollviewer.tag>                 <system:double>0.0</system:double>             </scrollviewer.tag>             <scrollviewer.height>                 <multibinding converter="{staticresource multiplyconverter}">                     <binding path="actualheight" elementname="expandercontent"/>                     <binding path="tag" relativesource="{relativesource self}" />                 </multibinding>             </scrollviewer.height>             <contentpresenter x:name="expandercontent" contentsource="content"/>         </scrollviewer>     </dockpanel>     <controltemplate.triggers>         <trigger property="isexpanded" value="true">             <trigger.enteractions>                 <beginstoryboard>                     <storyboard>                         <doubleanimation storyboard.targetname="expandercontentscrollview"                                              storyboard.targetproperty="tag"                                                                                               to="1"                                              duration="0:0:0.4"/>                     </storyboard>                 </beginstoryboard>             </trigger.enteractions>             <trigger.exitactions>                 <beginstoryboard>                     <storyboard>                         <doubleanimation storyboard.targetname="expandercontentscrollview"                                              storyboard.targetproperty="tag"                                                                                               to="0"                                              duration="0:0:0.4"/>                     </storyboard>                 </beginstoryboard>             </trigger.exitactions>         </trigger>     </controltemplate.triggers> </controltemplate> 

the template toggle button:

    <controltemplate x:key="moreinformationexpanderbutton" targettype="{x:type togglebutton}">     <grid >         <textblock name="tb" foreground="{dynamicresource bluesolidbrush}" fontsize="14" textdecorations="underline"/>     </grid>      <controltemplate.triggers>         <trigger property="ischecked" value="true">             <setter property="text" value="less information"  targetname="tb" />         </trigger>         <trigger property="ischecked" value="false">             <setter property="text" value="more information" targetname="tb" />         </trigger>         <trigger property="ismouseover" value="true">             <setter property="cursor" value="hand" />         </trigger>     </controltemplate.triggers> </controltemplate> 

the multiply converter:

public object convert(object[] values, type targettype, object parameter, cultureinfo culture)     {         double result = 1.0;         (int = 0; < values.length; i++)         {             if (values[i] double)             {                 result *= (double)values[i];             }         }          return result;     } 

exception , stack trace

an unhandled exception occurred, , application terminating. error details: cannot animate 'tag' property on 'system.windows.controls.scrollviewer' using 'system.windows.media.animation.doubleanimation'. details see inner exception.

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at system.windows.media.animation.doubleanimationbase.getcurrentvalue(object defaultoriginvalue, object defaultdestinationvalue, animationclock animationclock) @ system.windows.media.animation.animationlayer.getcurrentvalue(object defaultdestinationvalue) @ system.windows.media.animation.animationstorage.getcurrentpropertyvalue(animationstorage storage, dependencyobject d, dependencyproperty dp, propertymetadata metadata, object basevalue) @ system.windows.media.animation.animationstorage.oncurrenttimeinvalidated(object sender, eventargs args)