Error executing a LINQ query in C#

I am trying run the following script in code.     Note  that only LINQ query in LINQPad has no problem and generating the expected results. 

var zzz = (from aa in _DBService.AvailableStocks
join bb in _DBService.UserSelectedStocks.Where(c => c.UserID == UserID)
on aa.StocksID equals bb.StocksID into newgroup
from cc in newgroup.DefaultIfEmpty()
select new
{
StocksID = aa.StocksID,
Stock = aa.Stock,
UserID = (cc.UserID == null ? "Null" : cc.UserID.ToString()),
UserStockID = cc.UserStockID.ToString() == "" ? "Null" : cc.UserStockID.ToString(),
StockID = (cc.StocksID == null ? "Null" : cc.StocksID.ToString())
}).ToList();

The error is 

An exception of type 'System.NullReferenceException' occurred in Microsoft.EntityFrameworkCore.dll but was not handled in user code

Additional information: Object reference not set to an instance of an object.

Any help 

Shahzad