Fallback on default window for Linux.

gyp/sources.txt

@@ -28,6 +28,8 @@
 <(src_loc)/ui/layers/layer_manager.h
 <(src_loc)/ui/layers/layer_widget.cpp
 <(src_loc)/ui/layers/layer_widget.h
+<(src_loc)/ui/platform/linux/ui_window_linux.cpp
+<(src_loc)/ui/platform/linux/ui_window_linux.h
 <(src_loc)/ui/platform/linux/ui_utility_linux.cpp
 <(src_loc)/ui/platform/linux/ui_utility_linux.h
 <(src_loc)/ui/platform/mac/ui_window_mac.h

ui/platform/linux/ui_window_linux.cpp

@@ -0,0 +1,18 @@
+// This file is part of Desktop App Toolkit,
+// a set of libraries for developing nice desktop applications.
+//
+// For license and copyright information please follow this link:
+// https://github.com/desktop-app/legal/blob/master/LEGAL
+//
+#include "ui/platform/linux/ui_window_linux.h"
+
+namespace Ui {
+namespace Platform {
+
+std::unique_ptr<BasicWindowHelper> CreateWindowHelper(
+		not_null<RpWidget*> window) {
+	return nullptr;
+}
+
+} // namespace Platform
+} // namespace Ui

ui/platform/linux/ui_window_linux.h

@@ -0,0 +1,9 @@
+// This file is part of Desktop App Toolkit,
+// a set of libraries for developing nice desktop applications.
+//
+// For license and copyright information please follow this link:
+// https://github.com/desktop-app/legal/blob/master/LEGAL
+//
+#pragma once
+
+#include "ui/platform/ui_platform_window.h"