c# - Application crashes on changing Application Bar visibility -

i creating panorama application , want custumize application bar, visible single page. have created application bar in xaml:

<phone:phoneapplicationpage.applicationbar>     <shell:applicationbar x:name="appbar_opere" isvisible="false" ismenuenabled="true" mode="default" opacity="0.95" >          <shell:applicationbariconbutton iconuri="/assets/appbar/feature.search.png" text="cauta" />         <shell:applicationbariconbutton iconuri="/assets/appbar1/favs.png" text="favorite" />         <shell:applicationbariconbutton iconuri="/assets/appbar2/feature.settings.png" text="setari" />          <shell:applicationbar.menuitems>             <shell:applicationbarmenuitem text="clasa ix-a" />             <shell:applicationbarmenuitem text="clasa x-ea" />             <shell:applicationbarmenuitem text="clasa xi-ea" />             <shell:applicationbarmenuitem text="clasa xii-ea" />         </shell:applicationbar.menuitems>      </shell:applicationbar> </phone:phoneapplicationpage.applicationbar> 

then, using selectionchanged event, writen code:

private void panorama_selectionchanged(object sender, selectionchangedeventargs e)     {         switch(((panorama)sender).selectedindex)         {             case 1:                 appbar_opere.isvisible = true;                 break;             default:                 appbar_opere.isvisible = false;                 break;         }     } 

ok. when navigate page want applicationbar visible, app crashes, "object reference not set instance of object" on line:

appbar_opere.isvisible = true; 

why?

applicationbar not silverlight control, can't access way. correctly code is:

private void panorama_selectionchanged(object sender, selectionchangedeventargs e) {     switch(((panorama)sender).selectedindex)     {          case 1:              applicationbar.isvisible = true;              break;          default:              applicationbar.isvisible = false;              break;     } }