[Bug] .NET API bug GenerateExtensionMethods NullReferenceException

**Describe the bug**
When running `docfx docfx.json --serve` on a project containing a generic extension method with `allows ref struct`, DocFX throws a NullReferenceException.

**To Reproduce**
1) Create .NET 9 project, paste this code:
```csharp
namespace MyNamespace;

public static class TestClass
{
    public ref struct TestStruct;
    public static byte TestMethod<TRefStruct>(this TRefStruct value) where TRefStruct : allows ref struct => 1;
}
```
2) Initialize docfx with `docfx init`
3) Run `docfx docfx.json --serve`

(or use this C# project as playground)
[**Playground.zip**](https://github.com/user-attachments/files/20016991/Playground.zip)

**Expected behavior**
Generated API.

**Context (please complete the following information):**
- OS: Windows 11
- Docfx version: 2.78.3
- .NET version: .NET 9.0

- `docfx.json` config

```json
{
  "$schema": "https://raw.githubusercontent.com/dotnet/docfx/main/schemas/docfx.schema.json",
  "metadata": [
    {
      "src": [
        {
          "src": "../",
          "files": [
            "**/*.csproj"
          ]
        }
      ],
      "dest": "api"
    }
  ],
  "build": {
    "content": [
      {
        "files": [
          "**/*.{md,yml}"
        ],
        "exclude": [
          "_site/**"
        ]
      }
    ],
    "resource": [
      {
        "files": [
          "images/**"
        ]
      }
    ],
    "output": "_site",
    "template": [
      "default",
      "modern"
    ],
    "globalMetadata": {
      "_appName": "mysite",
      "_appTitle": "mysite",
      "_enableSearch": true,
      "pdf": true
    }
  }
}
```

- Exceptions

```csharp
NullReferenceException: Object reference not set to an instance of an object.
  at bool CheckBasicConstraints(Symbol containingSymbol, in CheckConstraintsArgs args, TypeParameterSymbol typeParameter, TypeWithAnnotations typeArgument, ArrayBuilder<TypeParameterDiagnosticInfo> diagnosticsBuilder, ArrayBuilder<TypeParameterDiagnosticInfo> nullabilityDiagnosticsBuilderOpt, ref
     ArrayBuilder<TypeParameterDiagnosticInfo> useSiteDiagnosticsBuilder)
  at bool CheckConstraints(Symbol containingSymbol, in CheckConstraintsArgs args, TypeMap substitution, TypeParameterSymbol typeParameter, TypeWithAnnotations typeArgument, ArrayBuilder<TypeParameterDiagnosticInfo> diagnosticsBuilder, ArrayBuilder<TypeParameterDiagnosticInfo> nullabilityDiagnosticsBuilderOpt, ref
     ArrayBuilder<TypeParameterDiagnosticInfo> useSiteDiagnosticsBuilder, HashSet<TypeParameterSymbol> ignoreTypeConstraintsDependentOnTypeParametersOpt)
  at bool CheckConstraints(Symbol containingSymbol, in CheckConstraintsArgs args, TypeMap substitution, ImmutableArray<TypeParameterSymbol> typeParameters, ImmutableArray<TypeWithAnnotations> typeArguments, ArrayBuilder<TypeParameterDiagnosticInfo> diagnosticsBuilder, ArrayBuilder<TypeParameterDiagnosticInfo>
     nullabilityDiagnosticsBuilderOpt, ref ArrayBuilder<TypeParameterDiagnosticInfo> useSiteDiagnosticsBuilder, BitVector skipParameters, HashSet<TypeParameterSymbol> ignoreTypeConstraintsDependentOnTypeParametersOpt)
  at MethodSymbol InferExtensionMethodTypeArguments(MethodSymbol method, TypeSymbol thisType, CSharpCompilation compilation, ref CompoundUseSiteInfo<AssemblySymbol> useSiteInfo, out bool wasFullyInferred)
  at MethodSymbol Create(MethodSymbol method, TypeSymbol receiverType, CSharpCompilation compilation, out bool wasFullyInferred)
  at MethodSymbol ReduceExtensionMethod(TypeSymbol receiverType, CSharpCompilation compilation, out bool wasFullyInferred)
  at MethodSymbol ReduceExtensionMethod(TypeSymbol receiverType, CSharpCompilation compilation)
  at IMethodSymbol ReduceExtensionMethod(ITypeSymbol receiverType)
  at void GenerateExtensionMethods(INamedTypeSymbol symbol, MetadataItem item) in SymbolVisitorAdapter.cs:640
  at MetadataItem VisitNamedType(INamedTypeSymbol symbol) in SymbolVisitorAdapter.cs:153
  at TResult Accept<TResult>(SymbolVisitor<TResult> visitor)
  at TResult Accept<TResult>(SymbolVisitor<TResult> visitor)
  at List<MetadataItem> VisitDescendants<T>(IEnumerable<T> children, Func<T, IEnumerable<T>> getChildren, Func<T, bool> filter) in SymbolVisitorAdapter.cs:512
  at MetadataItem VisitNamespace(INamespaceSymbol symbol) in SymbolVisitorAdapter.cs:133
  at TResult Accept<TResult>(SymbolVisitor<TResult> visitor)
  at TResult Accept<TResult>(SymbolVisitor<TResult> visitor)
  at List<MetadataItem> VisitDescendants<T>(IEnumerable<T> children, Func<T, IEnumerable<T>> getChildren, Func<T, bool> filter) in SymbolVisitorAdapter.cs:512
  at MetadataItem VisitAssembly(IAssemblySymbol symbol) in SymbolVisitorAdapter.cs:117
  at TResult Accept<TResult>(SymbolVisitor<TResult> visitor)
  at TResult Accept<TResult>(SymbolVisitor<TResult> visitor)
  at void CreateManagedReference((ValueTuple<IAssemblySymbol, Compilation> symbol) assemblies, ExtractMetadataConfig config, DotnetApiOptions options) in DotnetApiCatalog.ManagedReference.cs:25
  at async Task <Exec>g__Build|6_0(ExtractMetadataConfig config, DotnetApiOptions options) in DotnetApiCatalog.cs:111
  at async Task Exec(MetadataJsonConfig config, DotnetApiOptions options, string configDirectory, string outputDirectory) in DotnetApiCatalog.cs:71
  at void <Execute>b__0() in DefaultCommand.cs:45
  at int Run(LogOptions options, Action run) in CommandHelper.cs:48
  at int Execute(CommandContext context, Options options, CancellationToken cancellationToken) in DefaultCommand.cs:31
  at int Execute(CommandContext context, TSettings settings) in CancellableCommandBase.cs:24
  at Task<int> Execute(CommandContext context, CommandSettings settings) in CommandOfT.cs:40
  at async Task<int> Execute(CommandTree leaf, CommandTree tree, CommandContext context, ITypeResolver resolver, IConfiguration configuration) in CommandExecutor.cs:166
```

- .NET info

```csharp
.NET SDK:
 Version:           9.0.300-preview.0.25177.5
 Commit:            3d5b396331
 Workload version:  9.0.300-manifests.1e5233e8
 MSBuild version:   17.14.0-preview-25175-08+5880e1c75

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.22631
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\9.0.300-preview.0.25177.5\

.NET workloads installed:
 [tvos]
   Installation Source: VS 17.14.36017.23
   Manifest Version:    18.4.9288/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.tvos\18.4.9288\WorkloadManifest.json
   Install Type:              Msi

 [macos]
   Installation Source: VS 17.14.36017.23
   Manifest Version:    15.4.9288/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.macos\15.4.9288\WorkloadManifest.json
   Install Type:              Msi

 [android]
   Installation Source: VS 17.14.36017.23
   Manifest Version:    35.0.61/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.android\35.0.61\WorkloadManifest.json
   Install Type:              Msi

 [wasm-tools]
   Installation Source: VS 17.14.36017.23
   Manifest Version:    9.0.4/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.workload.mono.toolchain.current\9.0.4\WorkloadManifest.json
   Install Type:              Msi

 [aspire]
   Installation Source: VS 17.14.36017.23
   Manifest Version:    8.2.2/8.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\8.0.100\microsoft.net.sdk.aspire\8.2.2\WorkloadManifest.json
   Install Type:              Msi

 [maui-windows]
   Installation Source: VS 17.14.36017.23
   Manifest Version:    9.0.14/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maui\9.0.14\WorkloadManifest.json
   Install Type:              Msi

 [maccatalyst]
   Installation Source: VS 17.14.36017.23
   Manifest Version:    18.4.9288/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maccatalyst\18.4.9288\WorkloadManifest.json
   Install Type:              Msi

 [ios]
   Installation Source: VS 17.14.36017.23
   Manifest Version:    18.4.9288/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.ios\18.4.9288\WorkloadManifest.json
   Install Type:              Msi

Configured to use loose manifests when installing new manifests.

Host:
  Version:      9.0.4
  Architecture: x64
  Commit:       f57e6dc747

.NET SDKs installed:
  9.0.105 [C:\Program Files\dotnet\sdk]
  9.0.203 [C:\Program Files\dotnet\sdk]
  9.0.300-preview.0.25177.5 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 8.0.14 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 8.0.14 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 8.0.14 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  Not set

global.json file:
  Not found

Learn more:
  https://aka.ms/dotnet/info

Download .NET:
  https://aka.ms/dotnet/download
```


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[Bug] .NET API bug GenerateExtensionMethods NullReferenceException #10655

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[Bug] .NET API bug GenerateExtensionMethods NullReferenceException #10655

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