unhandled exception - photo chooser task windows phone 7.1 -

i need take picture directory of windows phone in application add child windows when click on text in child windows add button , called

public partial class annotationcontrol : childwindow {

    public observablecollection<string> cercle { get; set; }     public annotationcontrol()     {         initializecomponent();       }        private void childwindow_closing_1(object sender, system.componentmodel.canceleventargs e)     {      }      private void btnsave_click_1(object sender, routedeventargs e)     {                    this.dialogresult = true;     }        private void btncancel_click_1(object sender, routedeventargs e)     {         this.dialogresult = true;     }        private void browse_click(object sender, routedeventargs e)     { photochoosertask objphotochooser = new photochoosertask();                  objphotochooser.completed += new eventhandler<photoresult>(photochoosecall);                  objphotochooser.show();      } 

but when clicked on button choose picture application crashed "application_unhandledexception" 1 have idea please

use may helpful

    private void changeimage(object sender, routedeventargs e)     {          photochoosertask photochoosertask = new photochoosertask();         photochoosertask.completed += new eventhandler<photoresult>(photochoosertask_completed);         photochoosertask.show();     } void photochoosertask_completed(object sender, photoresult e)     {         if (e.taskresult == taskresult.ok)         {              system.windows.media.imaging.bitmapimage bmp = new system.windows.media.imaging.bitmapimage();             bmp.setsource(e.chosenphoto);              imagetapped.source = bmp;          }     }