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!

Reproduce the issue in Pinta 2.1.1 on Debian 12 with the Flatpak sandbox by opening an image outside the user directory and choosing Save as. Start at SaveDocumentImplmentationAction.SaveFileAs(Document document), then follow SaveDocumentAsAction and FileActions.RaiseSaveDocument; done means Save as no longer throws for the /run/user//doc/ path while ordinary Save still works.

Written by the indexing model from the issue text.