I found the soultion. The problem is the ID of UserControlbeing added. I have modified two methods AddUserSettings and AddSpaceSettings in the following way:

private void AddUserSettings() {
	UserSettings settings = LoadControl("~/UserControls/UserSettings.ascx") as UserSettings;
	settings.ID = "usersetting";
	AddSettings(settings);
}

private void AddSpaceSettings() {
	SpaceSettings settings = LoadControl("~/UserControls/SpaceSettings.ascx") as SpaceSettings;
	settings.ID = "spacesettings";
	AddSettings(settings);
}

Now it is working.