In the static constructor of ViewModelLocator class the SimpleIoc.Default.Register(); fails at IsInDesignModeStatic mode, if
ViewModelLocator
SimpleIoc.Default.Register();
IsInDesignModeStatic
if (ViewModelBase.IsInDesignModeStatic) { // put these lines here: if (SimpleIoc.Default.IsRegistered()) { SimpleIoc.Default.Unregister(); } SimpleIoc.Default.Register(); } else { SimpleIoc.Default.Register(); }