Enable nullability in Content.Client (#3257)

* Enable nullability in Content.Client

* Remove #nullable enable

* Merge fixes

* Remove Debug.Assert

* Merge fixes

* Fix build

* Fix build

Content.Client/EscapeMenuOwner.cs

@@ -16,7 +16,7 @@ namespace Content.Client
         [Dependency] private readonly IStateManager _stateManager = default!;
         [Dependency] private readonly IGameHud _gameHud = default!;
 
-        private EscapeMenu _escapeMenu;
+        private EscapeMenu? _escapeMenu;
 
         public void Initialize()
         {
@@ -35,12 +35,12 @@ namespace Content.Client
                 _escapeMenu.OnClose += () => _gameHud.EscapeButtonDown = false;
 
                 _inputManager.SetInputCommand(EngineKeyFunctions.EscapeMenu,
-                    InputCmdHandler.FromDelegate(s => Enabled()));
+                    InputCmdHandler.FromDelegate(_ => Enabled()));
             }
             else if (obj.OldState is GameScreenBase)
             {
                 // Switched FROM GameScreen.
-                _escapeMenu.Dispose();
+                _escapeMenu?.Dispose();
                 _escapeMenu = null;
 
                 _inputManager.SetInputCommand(EngineKeyFunctions.EscapeMenu, null);
@@ -49,7 +49,7 @@ namespace Content.Client
 
         private void Enabled()
         {
-            if (_escapeMenu.IsOpen)
+            if (_escapeMenu != null && _escapeMenu.IsOpen)
             {
                 if (_escapeMenu.IsAtFront())
                 {
@@ -71,12 +71,12 @@ namespace Content.Client
             if (value)
             {
                 _gameHud.EscapeButtonDown = true;
-                _escapeMenu.OpenCentered();
+                _escapeMenu?.OpenCentered();
             }
             else
             {
                 _gameHud.EscapeButtonDown = false;
-                _escapeMenu.Close();
+                _escapeMenu?.Close();
             }
         }
     }