Shows error when tries to save a saved file with the same file name after few edits in the file.

Pinta displays an error when attempting to save an already saved file that has been modified with a few changes. It does not permit the user to save the file, and consequently, the file is not created.
The text of the error message is:-
Unhandled exception:
Exception has been thrown by the target of an invocation
Details of the error:
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.ArgumentNullException: Value cannot be null. (Parameter 'key')
   at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Pinta.Actions.SaveDocumentImplmentationAction.SaveFileAs(Document document) in D:\a\Pinta\Pinta\Pinta\Actions\File\SaveDocumentImplementationAction.cs:line 126
   at Pinta.Actions.SaveDocumentImplmentationAction.Activated(Object sender, DocumentCancelEventArgs e) in D:\a\Pinta\Pinta\Pinta\Actions\File\SaveDocumentImplementationAction.cs:line 56
   at Pinta.Core.FileActions.RaiseSaveDocument(Document document, Boolean saveAs) in D:\a\Pinta\Pinta\Pinta.Core\Actions\FileActions.cs:line 119
   at Pinta.Core.Document.Save(Boolean saveAs) in D:\a\Pinta\Pinta\Pinta.Core\Classes\Document.cs:line 394
   at Pinta.Actions.SaveDocumentAsAction.Activated(Object sender, EventArgs e) in D:\a\Pinta\Pinta\Pinta\Actions\File\SaveDocumentAsAction.cs:line 48
   at Pinta.Core.Command.<.ctor>b__33_0(Object o, ActivatedArgs args) in D:\a\Pinta\Pinta\Pinta.Core\Actions\Command.cs:line 61
   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)