Return null if the file does not exist when opening a stream

binding/Binding/SKData.cs

@@ -92,7 +92,11 @@ namespace SkiaSharp
 				throw new ArgumentException ("The filename cannot be empty.", nameof (filename));
 
 			using (var stream = SKFileStream.OpenStream (filename)) {
-				return Create (stream);
+				if (stream == null) {
+					return null;
+				} else {
+					return Create (stream);
+				}
 			}
 		}
 

binding/Binding/SKStream.cs

@@ -216,6 +216,9 @@ namespace SkiaSharp
 
 		public static SKStreamAsset OpenStream (string path)
 		{
+			if (!File.Exists (path)) {
+				return null;
+			}
 			if (!IsPathSupported (path)) {
 				return new SKManagedStream (File.OpenRead (path), true);
 			} else {

binding/Binding/SKTypeface.cs

@@ -56,7 +56,11 @@ namespace SkiaSharp
 			if (path == null)
 				throw new ArgumentNullException (nameof (path));
 			using (var stream = SKFileStream.OpenStream (path)) {
-				return FromStream (stream, index);
+				if (stream == null) {
+					return null;
+				} else {
+					return FromStream (stream, index);
+				}
 			}
 		}
 

tests/Tests/SKTypefaceTest.cs

@@ -23,7 +23,7 @@ namespace SkiaSharp.Tests
 		};
 
 		[SkippableFact]
-		public void NullInWrongFileName()
+		public void NullWithMissingFile()
 		{
 			Assert.Null(SKTypeface.FromFile(Path.Combine(PathToFonts, "font that doesn't exist.ttf")));
 		}