Fix crash from background access to style::icon-s.

Telegram/SourceFiles/window/window_session_controller.cpp

@@ -1632,6 +1632,7 @@ void SessionController::cacheChatTheme(
 			this,
 			result = std::make_shared<Ui::ChatTheme>(std::move(descriptor))
 		]() mutable {
+			result->finishCreateOnMain();
 			cacheChatThemeDone(std::move(result));
 		});
 	});