Closing Espera with a mobile song in the playlist crashes Espera

[flagbug]

The stacktrace:

 at Espera.Core.Management.LibrarySerializer.SerializeSong(Song song)
   at Espera.Core.Management.LibrarySerializer.<Serialize>b__3(Object , PlaylistEntry entry)
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType)
   at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType)
   at Newtonsoft.Json.Linq.JToken.FromObjectInternal(Object o, JsonSerializer jsonSerializer)
   at Newtonsoft.Json.Linq.JObject.FromObject(Object o, JsonSerializer jsonSerializer)
   at Espera.Core.Management.LibrarySerializer.Serialize(IEnumerable`1 songs, IEnumerable`1 playlists, String songSourcePath, Stream targetStream)
   at Espera.Core.Management.LibraryFileWriter.Write(IEnumerable`1 songs, IEnumerable`1 playlists, String songSourcePath)
   at Espera.Core.Management.Library.<>c__DisplayClass29.<Save>b__32()
   at Espera.Core.Utility.Retry(Action block, Int32 retries)
   at Espera.Core.Management.Library.Save()
   at Espera.View.ViewModels.ShellViewModel.Dispose()
   at Espera.View.Views.ShellView.MetroWindowClosing(Object sender, CancelEventArgs e)
   at System.ComponentModel.CancelEventHandler.Invoke(Object sender, CancelEventArgs e)
   at System.Windows.Window.OnClosing(CancelEventArgs e)
   at System.Windows.Window.WmClose()
   at System.Windows.Window.WindowFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.PublicHooksFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)

We should remove temporary (mobile) songs before saving the playlist