Crash when saving as a new filename

[tester]

I am using Pinta as a Flatpak under Linux.

This issue occurs very often (but not always) when I do the following:
- open an existing JPG
- resize this image
- click on "Save As" and enter a new filename

It doesn't occur when I save under the same (existing) filename.

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at Pinta.Actions.SaveDocumentImplmentationAction.SaveFileAs(Document document)
   at Pinta.Actions.SaveDocumentImplmentationAction.Activated(Object sender, DocumentCancelEventArgs e)
   at Pinta.Core.FileActions.RaiseSaveDocument(Document document, Boolean saveAs)
   at Pinta.Core.Document.Save(Boolean saveAs)
   at Pinta.Actions.SaveDocumentAsAction.Activated(Object sender, EventArgs e)
   at InvokeStub_ActivatedHandler.Invoke(Object, Object, IntPtr*)
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)
   --- End of inner exception stack trace ---
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at GLib.Signal.ClosureInvokedCB(Object o, ClosureInvokedArgs args)
   at GLib.SignalClosure.MarshalCallback(IntPtr raw_closure, IntPtr return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr invocation_hint, IntPtr marshal_data)

[Cameron White (cameronwhite91)]

Confirmed as mentioned in https://github.com/flathub/com.github.PintaProject.Pinta/issues/34. This is probably some issue related to sandboxing