Use a fork of flutter_emoji

It has been ported to null safety, the original repo seems to be
abandoned - https://github.com/petehouston/flutter-emoji/pull/16

pubspec.lock

@@ -354,10 +354,12 @@ packages:
   flutter_emoji:
     dependency: "direct main"
     description:
-      name: flutter_emoji
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.2.1+1"
+      path: "."
+      ref: HEAD
+      resolved-ref: "2e2da41abf6a928206f44707fdf4d6149f049463"
+      url: "https://github.com/cek-cek/flutter-emoji"
+    source: git
+    version: "2.4.0"
   flutter_launcher_icons:
     dependency: "direct dev"
     description:

pubspec.yaml

@@ -41,7 +41,8 @@ dependencies:
   firebase_analytics: ^7.1.1
   flutter_cache_manager: ^2.1.2
   flutter_email_sender: ^3.0.1
-  flutter_emoji: ">= 2.0.0"
+  flutter_emoji:
+    git: https://github.com/cek-cek/flutter-emoji
   flutter_markdown: ^0.6.1
   flutter_runtime_env: ^0.0.4
   flutter_staggered_grid_view: ^0.3.0