Add null reference checks.

MD5Context.cs

@@ -74,6 +74,9 @@ public sealed class Md5Context : IChecksum
         _provider.TransformFinalBlock(Array.Empty<byte>(), 0, 0);
         var md5Output = new StringBuilder();
 
+        if(_provider.Hash is null)
+            return null;
+
         foreach(byte h in _provider.Hash)
             md5Output.Append(h.ToString("x2"));
 

SHA1Context.cs

@@ -74,6 +74,9 @@ public sealed class Sha1Context : IChecksum
         _provider.TransformFinalBlock(Array.Empty<byte>(), 0, 0);
         var sha1Output = new StringBuilder();
 
+        if(_provider.Hash is null)
+            return null;
+
         foreach(byte h in _provider.Hash)
             sha1Output.Append(h.ToString("x2"));
 

SHA256Context.cs

@@ -74,6 +74,9 @@ public sealed class Sha256Context : IChecksum
         _provider.TransformFinalBlock(Array.Empty<byte>(), 0, 0);
         var sha256Output = new StringBuilder();
 
+        if(_provider.Hash is null)
+            return null;
+
         foreach(byte h in _provider.Hash)
             sha256Output.Append(h.ToString("x2"));
 

SHA384Context.cs

@@ -74,6 +74,9 @@ public sealed class Sha384Context : IChecksum
         _provider.TransformFinalBlock(Array.Empty<byte>(), 0, 0);
         var sha384Output = new StringBuilder();
 
+        if(_provider.Hash is null)
+            return null;
+
         foreach(byte h in _provider.Hash)
             sha384Output.Append(h.ToString("x2"));
 

SHA512Context.cs

@@ -74,6 +74,9 @@ public sealed class Sha512Context : IChecksum
         _provider.TransformFinalBlock(Array.Empty<byte>(), 0, 0);
         var sha512Output = new StringBuilder();
 
+        if(_provider.Hash is null)
+            return null;
+
         foreach(byte h in _provider.Hash)
             sha512Output.Append(h.ToString("x2"));