OnCurrentSite() and OnSite(CMS.SiteProvider.SiteContext.CurrentSiteName) give System.NullReferenceEx

This is my first Kentico Project. Using Kentico 12 MVC. I am trying to use different sites for staging dev -> uat -> prod. And hence for queries I am trying to get documents of the current site. But whenever I use OnCurrentSite() or OnSite() it gives

"System.NullReferenceException: Object reference not set to an instance of an object. CMS.DocumentEngine.DocumentQueryBase`2.OnSite(SiteInfoIdentifier siteIdentifier, Boolean includeGlobal) +11".

I tried in IIS as well as Azure with the same result. Without the OnCurrentSite() or OnSite() I get the documents but it includes the document from all the sites which is not useful.

I am trying something simple like below.

      return CMS.DocumentEngine.DocumentHelper.GetDocuments<WhatWeDo>()
                .OnSite(CMS.SiteProvider.SiteContext.CurrentSiteName)
                .AddColumns("Logo" , "HeadingLine1", "HeadingLine2",)
                .TopN(1)
                .ToList()
                .Select(m => new WhatWeDoDto()
                {
                    Logo = m.Logo,
                    HeadingLine1 = m.HeadingLine1,
                    HeadingLine2 = m.HeadingLine2,
                })
                .FirstOrDefault();

Here are the results from SiteContext and HttpContext

var cSiteId = SiteContext.CurrentSiteID;
var cSite = SiteContext.CurrentSite;
var cName = CMS.SiteProvider.SiteContext.CurrentSiteName;
var host = HttpContext.Current.Request.Url.Host;
var domain = System.Web.HttpRuntime.AppDomainAppPath;
var sInfo = SiteInfoProvider.GetRunningSiteInfo(domain,host);

Result:

cSiteId 0   
cSite   null
cName   ""
host    "localhost"
domain  "C:\\Users\\abc\\Documents\\Name\\AzureDevops\\site\\site-DEV\\site\\"
sInfo   null

Any help would be appreciated.

Thank you