refactor: Switch to null safe sdk

lib/widgets/event_content/message.dart

@@ -125,7 +125,6 @@ class Message extends StatelessWidget {
                                       senderId: event.senderId,
                                       type: 'm.room.message',
                                       room: event.room,
-                                      roomId: event.roomId,
                                       status: EventStatus.sent,
                                       originServerTs: DateTime.now(),
                                     );

pubspec.lock

@@ -755,7 +755,7 @@ packages:
       name: matrix
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.6.2"
+    version: "0.7.0-nullsafety.3"
   matrix_api_lite:
     dependency: "direct overridden"
     description:

pubspec.yaml

@@ -45,7 +45,7 @@ dependencies:
   intl: any
   localstorage: ^4.0.0+1
   lottie: ^1.2.1
-  matrix: ^0.6.2
+  matrix: ^0.7.0-nullsafety.3
   matrix_link_text: ^1.0.2
   native_imaging:
     git: https://gitlab.com/famedly/libraries/native_imaging.git