Readme documentation says
// Android: Will open mail app or show native picker.
// iOS: Will open mail app if single mail app found.
var result = await OpenMailApp.openMailApp();
It should be
// Android: Will open mail app or show native picker.
// iOS: Will open mail app if single mail app found.
var result = await OpenMailAppPlus.openMailApp();
Readme documentation says
It should be