[WPF] Fix application crash when storage file was deleted while app is being active. (#897)

* Create file if non exist on each call

* Added asyn operation

Author: abodalevsky

Examples/CodePushDemoApp/windows/CodePushDemoApp/CodePushDemoApp.nuget.targets

@@ -62,11 +62,12 @@ public class ApplicationDataContainer
         private readonly SemaphoreSlim mutex = new SemaphoreSlim(1, 1);
 
         const string STORAGE_NAME = "AppStorage.data";
-        IFile storageFile = null;
+        string storageFileName = null;
 
         public ApplicationDataContainer(string name = STORAGE_NAME)
         {
-            storageFile = FileSystem.Current.LocalStorage.CreateFileAsync(name, CreationCollisionOption.OpenIfExists).Result;
+            storageFileName = name;
+            var storageFile = FileSystem.Current.LocalStorage.CreateFileAsync(storageFileName, CreationCollisionOption.OpenIfExists).Result;
             var data = CodePushUtils.GetJObjectFromFileAsync(storageFile).Result;
 
             if (data != null)
@@ -90,14 +91,16 @@ async Task SaveAsync()
         {
             await mutex.WaitAsync().ConfigureAwait(false);
             var jobject = JObject.FromObject(Values);
+            var storageFile = await FileSystem.Current.LocalStorage.CreateFileAsync(storageFileName, CreationCollisionOption.OpenIfExists).ConfigureAwait(false);
             await storageFile.WriteAllTextAsync(JsonConvert.SerializeObject(jobject)).ConfigureAwait(false);
             mutex.Release();
         }
 
         public async Task DeleteAsync()
         {
             Values.Clear();
-            await storageFile.DeleteAsync();
+            var storageFile = await FileSystem.Current.LocalStorage.CreateFileAsync(storageFileName, CreationCollisionOption.OpenIfExists).ConfigureAwait(false);
+            await storageFile.DeleteAsync().ConfigureAwait(false);
         }
     }
 }