Quick fix for "Mandatory WML child missing" error when starting MP

This isn't a proper fix since then the minimap doesn't show, but it at least allows
you to start an MP game.

Fixes #2724.

src/gui/dialogs/ingame_ui_base.cpp

@@ -27,7 +27,7 @@ namespace dialogs
 ingame_ui_base::ingame_ui_base()
 	: disp_(display::get_singleton())
 	, game_config_(game_config_manager::get()->game_config())
-	, scenario_(game_config_.child("scenario"))
+	, scenario_(game_config_.child_or_empty("scenario")) // FIXME: invalid in MP
 {
 }