Fix text flow arbitrary drawable wrapper accessing child in an unsafe manner

Closes #34126.

I'm not really sure how that issue could have ever happened to begin
with but I can see a way to make it hopefully safer. If it fails again
then it's clearly goblins.

Author: bdach

osu.Game/Graphics/Containers/OsuTextFlowContainer.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <[email protected]>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using osu.Framework.Extensions.IEnumerableExtensions;
 using osu.Framework.Graphics;
@@ -14,23 +12,27 @@ namespace osu.Game.Graphics.Containers
 {
     public partial class OsuTextFlowContainer : TextFlowContainer
     {
-        public OsuTextFlowContainer(Action<SpriteText> defaultCreationParameters = null)
+        public OsuTextFlowContainer(Action<SpriteText>? defaultCreationParameters = null)
             : base(defaultCreationParameters)
         {
         }
 
         protected override SpriteText CreateSpriteText() => new OsuSpriteText();
 
-        public ITextPart AddArbitraryDrawable(Drawable drawable) => AddPart(new TextPartManual(new ArbitraryDrawableWrapper { Child = drawable }.Yield()));
+        public ITextPart AddArbitraryDrawable(Drawable drawable) => AddPart(new TextPartManual(new ArbitraryDrawableWrapper(drawable).Yield()));
 
-        public ITextPart AddIcon(IconUsage icon, Action<SpriteText> creationParameters = null) => AddText(icon.Icon.ToString(), creationParameters);
+        public ITextPart AddIcon(IconUsage icon, Action<SpriteText>? creationParameters = null) => AddText(icon.Icon.ToString(), creationParameters);
 
         private partial class ArbitraryDrawableWrapper : Container, IHasLineBaseHeight
         {
-            public float LineBaseHeight => (Child as IHasLineBaseHeight)?.LineBaseHeight ?? DrawHeight;
+            private readonly IHasLineBaseHeight? lineBaseHeightSource;
+
+            public float LineBaseHeight => lineBaseHeightSource?.LineBaseHeight ?? DrawHeight;
 
-            public ArbitraryDrawableWrapper()
+            public ArbitraryDrawableWrapper(Drawable drawable)
             {
+                Child = drawable;
+                lineBaseHeightSource = drawable as IHasLineBaseHeight;
                 AutoSizeAxes = Axes.Both;
             }
         }

osu.Game/Screens/Menu/SupporterDisplay.cs

@@ -105,7 +105,7 @@ protected override void LoadComplete()
 
                     Schedule(() =>
                     {
-                        heart?.FlashColour(Color4.White, 750, Easing.OutQuint).Loop();
+                        heart.FlashColour(Color4.White, 750, Easing.OutQuint).Loop();
                     });
                 });
             }, true);