Fix assert in the generic file dialog in the dialogs sample

Cast the dialog pointer to wxFileDialogBase to avoid failures when using
wxGenericFileDialog, which inherits from wxFileDialogBase, but not from
wxFileDialog itself.

Closes #18506.

samples/dialogs/dialogs.cpp

@@ -1502,7 +1502,10 @@ private:
 
     void OnUpdateLabelUI(wxUpdateUIEvent& event)
     {
-        wxFileDialog* const dialog = wxStaticCast(GetParent(), wxFileDialog);
+        // In this sample, the dialog may be either wxFileDialog itself, or
+        // wxGenericFileDialog, so we need to cast to the base class. In a
+        // typical application, we would cast to just wxFileDialog instead.
+        wxFileDialogBase* const dialog = wxStaticCast(GetParent(), wxFileDialogBase);
         const wxString fn = dialog->GetCurrentlySelectedFilename();
 
         wxString msg;