refactor: Migrate to null safety

pubspec.yaml

@@ -4,7 +4,7 @@ publish_to: none
 version: 1.2.0+2041
 
 environment:
-  sdk: ">=2.11.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   adaptive_dialog: ^1.1.0
@@ -14,6 +14,7 @@ dependencies:
   blurhash_dart: ^1.1.0
   cached_network_image: ^3.1.0
   chewie: ^1.2.2
+  collection: ^1.15.0-nullsafety.4
   cupertino_icons: any
   desktop_drop: ^0.3.0
   desktop_notifications: ^0.6.1
@@ -35,8 +36,8 @@ dependencies:
     sdk: flutter
   flutter_map: ^0.14.0
   flutter_matrix_html: ^1.1.0
-  flutter_olm: ^1.1.2
-  flutter_openssl_crypto: ^0.0.1
+  flutter_olm: ^1.2.0
+  flutter_openssl_crypto: ^0.1.0
   flutter_secure_storage: ^5.0.2
   flutter_slidable: ^1.1.0
   flutter_svg: ^0.22.0