Server Error in '/' Application.

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 


[ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index]
   System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource) +72
   mini_ielts.business_logic.ContainerService.PopulateDropdown(IEnumerable`1 choices, String qnumber, List`1 answerPair, String properties, Boolean questionNumberInvisible) in C:\Users\admin\source\repos\mini-ielts\mini-ielts.business-logic\ContainerService.cs:480
   mini_ielts.business_logic.ContainerService.CompileHtml(Container arg, List`1 questions, List`1 answerPairs, IEnumerable`1 answerMarks, Boolean compileHtml) in C:\Users\admin\source\repos\mini-ielts\mini-ielts.business-logic\ContainerService.cs:233
   mini_ielts.business_logic.<>c__DisplayClass9_0.<GetContainersByTestId>b__2(Container x) in C:\Users\admin\source\repos\mini-ielts\mini-ielts.business-logic\ContainerService.cs:73
   System.Linq.WhereSelectEnumerableIterator`2.MoveNext() +223
   System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +524
   System.Linq.Enumerable.ToList(IEnumerable`1 source) +70
   mini_ielts.business_logic.ContainerService.GetContainersByTestId(Int32 testId, List`1 answerPairs, IEnumerable`1 answerMarks, Boolean compileHtml) in C:\Users\admin\source\repos\mini-ielts\mini-ielts.business-logic\ContainerService.cs:73
   mini_ielts.business_logic.Repos.TestsRepo.GetTest(Int32 testId, List`1 answerPairs, Boolean compileHtml) in C:\Users\admin\source\repos\mini-ielts\mini-ielts.business-logic\Repos\TestsRepo.cs:0
   mini_ielts.business_logic.TestService.GetTest(Int32 testId, List`1 answerPairs, Boolean compileHtml) in C:\Users\admin\source\repos\mini-ielts\mini-ielts.business-logic\TestService.cs:136
   mini_ielts.Controllers.TestsController.ViewSolution(String testId, String testType, String testName) +585
   lambda_method(Closure , ControllerBase , Object[] ) +238
   System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +229
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +35
   System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +39
   System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +71
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +42
   System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +72
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +386
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +42
   System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +38
   System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +186
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +38
   System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +29
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +67
   System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +53
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +36
   System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +38
   System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult, ProcessRequestState innerState) +44
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +67
   System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +38
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +399
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +157

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.6.1590.0