#1343 refactor(core, gui): replace nullable IMainWindow with default ConsoleWindow fallback

[Hiroshi Miura]

refactor(core, gui): replace nullable IMainWindow with default ConsoleWindow fallback - PR#2349
topic/miurahr/core/make-getmainwindow-non-null

This is refactor to make Core.getMainWindow Non-Null. It is always MainWindow object in real execution but could be nullable for test environment. Now I try to fallback to ConsoleWindow object and avoid null.
This make every call to be clean.