refactor: Make most of the utils null safe

lib/utils/room_send_file_extension.dart

@@ -1,20 +1,4 @@
-/*
- *   Famedly App
- *   Copyright (C) 2020 Famedly GmbH
- *
- *   This program is free software: you can redistribute it and/or modify
- *   it under the terms of the GNU Affero General Public License as
- *   published by the Free Software Foundation, either version 3 of the
- *   License, or (at your option) any later version.
- *
- *   This program is distributed in the hope that it will be useful,
- *   but WITHOUT ANY WARRANTY; without even the implied warranty of
- *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- *   GNU Affero General Public License for more details.
- *
- *   You should have received a copy of the GNU Affero General Public License
- *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
- */
+//@dart=2.12
 
 import 'package:matrix/matrix.dart';
 
@@ -23,12 +7,12 @@ import 'resize_image.dart';
 extension RoomSendFileExtension on Room {
   Future<Uri> sendFileEventWithThumbnail(
     MatrixFile file, {
-    String txid,
-    Event inReplyTo,
-    String editEventId,
-    bool waitUntilSent,
+    String? txid,
+    Event? inReplyTo,
+    String? editEventId,
+    bool? waitUntilSent,
   }) async {
-    MatrixFile thumbnail;
+    MatrixImageFile? thumbnail;
     if (file is MatrixImageFile) {
       thumbnail = await file.resizeImage();