Unhandled error message when using "save as" on files outside the user directory

When I open an image outside my user directory and then choose "save as" I get the following error message:
`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 Pinta.Core.Command.<.ctor>b__33_0(Object o, ActivatedArgs args)
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.Invoke(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)`

Problem seems to be the pre-selected path of the sandbox in the format "/run/user//doc/". A simple "save" works though! Operating system is Debian 12 with flatpak in version 1.14.4 and pinta in 2.1.1!

Hope you can reproduce and fix it. Thanks in advance!

Riprodurre il problema in Pinta 2.1.1 su Debian 12 con la sandbox di Flatpak aprendo un'immagine al di fuori della directory dell'utente e scegliendo Save as. Iniziare da SaveDocumentImplmentationAction.SaveFileAs(Document document), quindi seguire SaveDocumentAsAction e FileActions.RaiseSaveDocument; il lavoro è completato quando Save as non genera più un'eccezione per il percorso /run/user//doc/, mentre il normale Save continua a funzionare.

Scritto dal modello di indicizzazione a partire dal testo della issue.