TF-2537 Fix app crash when user trying attach file to composer in web app

Signed-off-by: dab246 <tdvu@linagora.com>

core/lib/utils/web_renderer/canvas_kit.dart

@@ -0,0 +1 @@
+export 'canvas_kit_stub.dart' if (dart.library.html) 'canvas_kit_web.dart';

core/lib/utils/web_renderer/canvas_kit_stub.dart

@@ -0,0 +1,4 @@
+/// Whether the CanvasKit renderer is being used on web.
+///
+/// Always returns `false` on non-web.
+bool get isRendererCanvasKit => false;

core/lib/utils/web_renderer/canvas_kit_web.dart

@@ -0,0 +1,6 @@
+import 'dart:js' as js;
+
+/// Whether the CanvasKit renderer is being used on web.
+///
+/// Always returns `false` on non-web.
+bool get isRendererCanvasKit => js.context['flutterCanvasKit'] != null;