NullReferenceException on downloading file from SkyDrive at Windows Phone 7 app

NullReferenceException on downloading file from SkyDrive at Windows Phone
7 app

I have app that should backup and restore it's database to SkyDrive. But
if I close app on uploading or downloading backup to/from SkyDrive using
"start" button (Fast Application Switching) and then return to app using
"back" button I catch unhandled NullReferenceException:
void RestoreFromSkyDrive() {
ShowProgress();
LiveConnectClient client = new LiveConnectClient(App.Session);
string id = string.Empty;
if (client != null) {
client.GetCompleted += (obj, args) => {
try {
List<object> items;
if (args != null && args.Result != null && args.Result["data"] !=
null)
items = args.Result["data"] as List<object>;
else
return;
foreach (object item in items) {
Dictionary<string, object> file = item as Dictionary<string,
object>;
// fileName is a name of backup-file
if (file != null && file["name"] != null &&
file["name"].ToString() == fileName) {
id = file["id"].ToString();
if (client != null) client.DownloadCompleted += new
EventHandler<LiveDownloadCompletedEventArgs>(client_DownloadCompleted);
if (client != null)
client.DownloadAsync(String.Format("{0}/content", id));
break;
}
}
}
catch (Exception ex) {
MessageBox.Show(ex.Message + " restore");
}
};
if (client != null) {
client.GetAsync("me/skydrive/files");
}
}
}
void client_DownloadCompleted(object sender,
LiveDownloadCompletedEventArgs e) {
try {
if (e.Error == null) {
Stream stream = e.Result;
using (IsolatedStorageFile storage =
IsolatedStorageFile.GetUserStoreForApplication()) {
var fileToSave = new IsolatedStorageFileStream(Constants.dbName,
FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None,
storage);
stream.CopyTo(fileToSave);
stream.Flush();
stream.Close();
fileToSave.Close();
}
}
else {
MessageBox.Show(e.Error.Message + " dcompleted mb");
}
}
catch (WebException ex) { MessageBox.Show(ex.Message + " dcompleted ex"); }
HideProgress();
}
I insert lot of checks for null but anyway I catch NullReferenceException.
I don't know what I can do with that.
PS: StackTrace with enabled CLR Exceptions:
System.Net.WebException occurred
Message=WebException
StackTrace:
at
System.Net.Browser.ClientHttpWebRequest.InternalEndGetResponse(IAsyncResult
asyncResult)
at
System.Net.Browser.ClientHttpWebRequest.<>c__DisplayClass2.<EndGetResponse>b__1(Object
sendState)
at
System.Net.Browser.AsyncHelper.<>c__DisplayClass4.<BeginOnUI>b__0(Object
sendState)
at System.Reflection.RuntimeMethodInfo.InternalInvoke(RuntimeMethodInfo
rtmi, Object obj, BindingFlags invokeAttr, Binder binder, Object
parameters, CultureInfo culture, Boolean isBinderDefault, Assembly
caller, Boolean verifyAccess, StackCrawlMark& stackMark)
at System.Reflection.RuntimeMethodInfo.InternalInvoke(Object obj,
BindingFlags invokeAttr, Binder binder, Object[] parameters,
CultureInfo culture, StackCrawlMark& stackMark)
at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
at System.Delegate.DynamicInvokeOne(Object[] args)
at System.MulticastDelegate.DynamicInvokeImpl(Object[] args)
at System.Delegate.DynamicInvoke(Object[] args)
at
System.Windows.Threading.Dispatcher.<>c__DisplayClass4.<FastInvoke>b__3()
at System.Reflection.RuntimeMethodInfo.InternalInvoke(RuntimeMethodInfo
rtmi, Object obj, BindingFlags invokeAttr, Binder binder, Object
parameters, CultureInfo culture, Boolean isBinderDefault, Assembly
caller, Boolean verifyAccess, StackCrawlMark& stackMark)
at System.Reflection.RuntimeMethodInfo.InternalInvoke(Object obj,
BindingFlags invokeAttr, Binder binder, Object[] parameters,
CultureInfo culture, StackCrawlMark& stackMark)
at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
at System.Delegate.DynamicInvokeOne(Object[] args)
at System.MulticastDelegate.DynamicInvokeImpl(Object[] args)
at System.Delegate.DynamicInvoke(Object[] args)
at System.Windows.Threading.DispatcherOperation.Invoke()
at System.Windows.Threading.Dispatcher.Dispatch(DispatcherPriority
priority)
at System.Windows.Threading.Dispatcher.OnInvoke(Object context)
at System.Windows.Hosting.CallbackCookie.Invoke(Object[] args)
at System.Windows.Hosting.DelegateWrapper.InternalInvoke(Object[] args)
at System.Windows.RuntimeHost.ManagedHost.InvokeDelegate(IntPtr
pHandle, Int32 nParamCount, ScriptParam[] pParams, ScriptParam&
pResult)
StackTrace with disabled CLR Exceptions:
System.NullReferenceException was unhandled
Message=NullReferenceException
StackTrace:
at
Microsoft.Live.Operations.DownloadOperation.CompleteOperation(Exception
error)
at Microsoft.Live.Operations.DownloadOperation.OnCancel()
at Microsoft.Live.Operations.WebOperation.OnGetWebResponse(IAsyncResult
ar)
at
System.Net.Browser.ClientHttpWebRequest.<>c__DisplayClassa.<InvokeGetResponseCallback>b__8(Object
state2)
at System.Threading.ThreadPool.WorkItem.WaitCallback_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext
executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadPool.WorkItem.doWork(Object o)
at System.Threading.Timer.ring()