Check for null arguments in GetArgumentDictionary

Addresses #5423

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/DefaultViewComponentHelper.cs

@@ -135,12 +135,15 @@ namespace Microsoft.AspNetCore.Mvc.ViewComponents
         // Internal for testing
         internal IDictionary<string, object> GetArgumentDictionary(ViewComponentDescriptor descriptor, object arguments)
         {
-            if (descriptor.Parameters.Count == 1 && descriptor.Parameters[0].ParameterType.IsAssignableFrom(arguments.GetType()))
+            if (arguments != null)
             {
-                return new Dictionary<string, object>(capacity: 1, comparer: StringComparer.OrdinalIgnoreCase)
+                if (descriptor.Parameters.Count == 1 && descriptor.Parameters[0].ParameterType.IsAssignableFrom(arguments.GetType()))
                 {
-                    { descriptor.Parameters[0].Name, arguments }
+                    return new Dictionary<string, object>(capacity: 1, comparer: StringComparer.OrdinalIgnoreCase)
-                };
+                    {
+                        { descriptor.Parameters[0].Name, arguments }
+                    };
+                }
             }
 
             return PropertyHelper.ObjectToDictionary(arguments);

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/ViewComponents/DefaultViewComponentHelperTest.cs

@@ -15,6 +15,21 @@ namespace Microsoft.AspNetCore.Mvc.ViewComponents
 {
     public class DefaultViewComponentHelperTest
     {
+        [Fact]
+        public void GetArgumentDictionary_SupportsNullArguments()
+        {
+            // Arrange
+            var helper = CreateHelper();
+            var descriptor = CreateDescriptorForType(typeof(ViewComponentSingleParam));
+
+            // Act
+            var argumentDictionary = helper.GetArgumentDictionary(descriptor, null);
+
+            // Assert
+            Assert.Equal(0, argumentDictionary.Count);
+            Assert.IsType(typeof(Dictionary<string,object>),argumentDictionary);
+        }
+
         [Fact]
         public void GetArgumentDictionary_SupportsAnonymouslyTypedArguments()
         {