Handle null in AssemblyVersion attribute

src/Compilers/CSharp/Portable/Symbols/Source/SourceAssemblySymbol.cs

@@ -2189,7 +2189,7 @@ private void DecodeWellKnownAttribute(ref DecodeWellKnownAttributeArguments<Attr
                 if (!VersionHelper.TryParseAssemblyVersion(verString, allowWildcard: !_compilation.IsEmitDeterministic, version: out version))
                 {
                     Location attributeArgumentSyntaxLocation = attribute.GetAttributeArgumentSyntaxLocation(0, arguments.AttributeSyntaxOpt);
-                    bool foundBadWildcard = _compilation.IsEmitDeterministic && verString.Contains('*');
+                    bool foundBadWildcard = _compilation.IsEmitDeterministic && verString?.Contains('*') == true;
                     arguments.Diagnostics.Add(foundBadWildcard ? ErrorCode.ERR_InvalidVersionFormatDeterministic : ErrorCode.ERR_InvalidVersionFormat, attributeArgumentSyntaxLocation);
                 }
 

src/Compilers/CSharp/Test/Emit/Attributes/AttributeTests.cs

@@ -25,6 +25,22 @@ public class AttributeTests : WellKnownAttributesTestBase
 
         #region Function Tests
 
+        [Fact, WorkItem(26464, "https://github.com/dotnet/roslyn/issues/26464")]
+        public void TestNullInAssemblyVersionAttribute()
+        {
+            var source = @"
+[assembly: System.Reflection.AssemblyVersionAttribute(null)]
+class Program
+{
+}";
+            var comp = CreateCompilation(source, options: TestOptions.DebugDll.WithDeterministic(true));
+            comp.VerifyDiagnostics(
+                // (2,55): error CS7034: The specified version string does not conform to the required format - major[.minor[.build[.revision]]]
+                // [assembly: System.Reflection.AssemblyVersionAttribute(null)]
+                Diagnostic(ErrorCode.ERR_InvalidVersionFormat, "null").WithLocation(2, 55)
+                );
+        }
+
         [Fact]
         [WorkItem(21194, "https://github.com/dotnet/roslyn/issues/21194")]
         public void TestQuickAttributeChecker()