Initialize TransformInfo array

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1FrameInfo.cs

@@ -60,6 +60,12 @@ internal partial class Av1FrameInfo
             {
                 Point point = new(x, y);
                 this.superblockInfos[i] = new(this, point);
+                for (int j = 0; j < this.modeInfoCountPerSuperblock; j++)
+                {
+                    this.transformInfosY[j] = new Av1TransformInfo();
+                    this.transformInfosUv[j] = new Av1TransformInfo();
+                }
+
                 i++;
             }
         }

src/ImageSharp/Formats/Heif/Av1/Tiling/Av1TransformInfo.cs

@@ -10,6 +10,14 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
 /// </summary>
 internal class Av1TransformInfo
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1TransformInfo"/> class.
+    /// </summary>
+    public Av1TransformInfo()
+        : this(Av1TransformSize.Size4x4, 0, 0)
+    {
+    }
+
     /// <summary>
     /// Initializes a new instance of the <see cref="Av1TransformInfo"/> class.
     /// </summary>