Null reference exception created on multiple api calls

I am facing an issue currently Developing Web API for a website which i need to connect to the cross platform app.

The code breaks when i call the API(only on multiple instances).

I have done enough of search on documentation, forums and on Stack Overflow. but there was no solution to my issue.

the code produces an exception at

var scope = EngineContext.Current.ContainerManager.Scope()  --Null reference exception

I have tried debugging the code but the exception is not generated regularly.

but i found some points where it breaks.

var sis = EngineContext.Current.Resolve<StoreInformationSettings>()

I have tried loading it once again, inside while statement as

var sis = EngineContext.Current.Resolve<StoreInformationSettings>()`
while(sis == null)
{
    sis = EngineContext.Current.Resolve<StoreInformationSettings>()
}


var webHelper = EngineContext.Current.Resolve<IWebHelper>();

even in here, i did the same thing which worked again

var webHelper = EngineContext.Current.Resolve<IWebHelper>();
while(webHelper == null)
{
    webHelper = EngineContext.Current.Resolve<IWebHelper>();
}
and similarly, a few more instances. this suggests, it's loading but after a delay!

I have tried updating the Autofac Nuget package as instructed in some forum which failed again(i rolled back the version afterwards).

I am using MVC 4
Visual Studio 2015
Autofac Version 3.5.2

this would be of great help, because I am currently stuck here. I cannot carry on further development because its breaking every now and then.
Thank you so much in advance for your help.