Merge pull request #7370 from AvaloniaUI/fixes/x11-transparency-crash

Another workaround for broken TopLevel.cs causing crash on X11

src/Avalonia.X11/X11Window.cs

@@ -341,9 +341,13 @@ namespace Avalonia.X11
 
         public Action<WindowTransparencyLevel> TransparencyLevelChanged
         {
-            get => _transparencyHelper.TransparencyLevelChanged;
-            set => _transparencyHelper.TransparencyLevelChanged = value;
-        }        
+            get => _transparencyHelper?.TransparencyLevelChanged;
+            set
+            {
+                if (_transparencyHelper != null)
+                    _transparencyHelper.TransparencyLevelChanged = value;
+            }
+        }
 
         public Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
 
@@ -1163,7 +1167,8 @@ namespace Avalonia.X11
         {
         }
 
-        public WindowTransparencyLevel TransparencyLevel => _transparencyHelper.CurrentLevel;
+        public WindowTransparencyLevel TransparencyLevel =>
+            _transparencyHelper?.CurrentLevel ?? WindowTransparencyLevel.None;
 
         public AcrylicPlatformCompensationLevels AcrylicCompensationLevels { get; } = new AcrylicPlatformCompensationLevels(1, 0.8, 0.8);