OpenApiSchemaBuilder.GetMethodUriWebInvoke can throw NullReferenceException

If swagger is turned on, then CoreWCF can make it fail to show its UI with exception `Object reference not set to an instance of an object.` Stack is
```
at CoreWCF.OpenApi.OpenApiSchemaBuilder.GetMethodUriWebInvoke(MethodInfo methodInfo)
at CoreWCF.OpenApi.OpenApiSchemaBuilder.PopulateOpenApiPath(OpenApiDocument document, MethodInfo methodInfo, IEnumerable`1 tagsToHide, String additionalBasePath, WebMessageFormat behaviorFormat, Func`2 getOperationInfo)
at CoreWCF.OpenApi.OpenApiSchemaBuilder.PopulateOpenApiPathsOperations(OpenApiDocument document, IEnumerable`1 contracts, IEnumerable`1 tagsToHide)
at CoreWCF.OpenApi.OpenApiSchemaBuilder.BuildOpenApiSpecificationDocument(OpenApiOptions info, IEnumerable`1 contracts)
at CoreWCF.OpenApi.OpenApiDocumentProvider.GetSwagger(String documentName, String host, String basePath)
at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware.g__Awaited|6_0(ExceptionHandlerMiddleware middleware, HttpContext context, Task task)
```

As this is a project where conversion from net framework to net core is a WIP, its likely I've not got everything right, but some clues in the error message would be helpful.