Add sanitation for dithering methods.

src/ImageSharp/Processing/Processors/Dithering/ErrorDither.cs

@@ -129,6 +129,11 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             where TPaletteDitherImageProcessor : struct, IPaletteDitherImageProcessor<TPixel>
             where TPixel : unmanaged, IPixel<TPixel>
         {
+            if (this == default)
+            {
+                ThrowDefaultInstance();
+            }
+
             float scale = processor.DitherScale;
             for (int y = bounds.Top; y < bounds.Bottom; y++)
             {

src/ImageSharp/Processing/Processors/Dithering/OrderedDither.cs

@@ -141,6 +141,11 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             where TPaletteDitherImageProcessor : struct, IPaletteDitherImageProcessor<TPixel>
             where TPixel : unmanaged, IPixel<TPixel>
         {
+            if (this == default)
+            {
+                ThrowDefaultInstance();
+            }
+
             int spread = CalculatePaletteSpread(processor.Palette.Length);
             float scale = processor.DitherScale;
 

tests/ImageSharp.Tests/Processing/Processors/Dithering/DitherTests.cs

@@ -43,6 +43,13 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Dithering
                 { KnownDitherings.Ordered3x3, nameof(KnownDitherings.Ordered3x3) }
             };
 
+        public static readonly TheoryData<IDither> DefaultInstanceDitherers
+            = new TheoryData<IDither>
+            {
+                default(ErrorDither),
+                default(OrderedDither)
+            };
+
         private static readonly ImageComparer ValidatorComparer = ImageComparer.TolerantPercentage(0.05f);
 
         private static IDither DefaultDitherer => KnownDitherings.Bayer4x4;
@@ -175,5 +182,18 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Dithering
                 c => c.Dither(dither),
                 name);
         }
+
+        [Theory]
+        [MemberData(nameof(DefaultInstanceDitherers))]
+        public void ShouldThrowForDefaultDitherInstance(IDither dither)
+        {
+            void Command()
+            {
+                using var image = new Image<Rgba32>(10, 10);
+                image.Mutate(x => x.Dither(dither));
+            }
+
+            Assert.Throws<ImageProcessingException>(Command);
+        }
     }
 }

tests/ImageSharp.Tests/Processing/Processors/Quantization/QuantizerTests.cs

@@ -155,10 +155,10 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Quantization
 
         public static readonly TheoryData<IDither> DefaultInstanceDitherers
             = new TheoryData<IDither>
-        {
-            default(ErrorDither),
-            default(OrderedDither)
-        };
+            {
+                default(ErrorDither),
+                default(OrderedDither)
+            };
 
         private static readonly ImageComparer ValidatorComparer = ImageComparer.TolerantPercentage(0.05F);