Error when deleting encrypted form submission

Description

Precondition: Encryption packages are installed.

Steps to reproduce

1. Create a Form then add it to a page.
2. Open the page in view mode.
3. Submit the form.
4. Open the form submission view.
5. Delete a submission data record.

Expected:
The record is deleted successfully.

Actual:
Exception is thrown.

Server Error in '/' Application.
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Object reference not set to an instance of an object.
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.NullReferenceException: Object reference not set to an instance of an object.

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:

[NullReferenceException: Object reference not set to an instance of an object.]
EPiServer.Forms.Core.Data.Internal.FormSession.get_Item(String name) +77
EPiServer.Forms.Core.Data.Internal.<LoadSubmissionFromStorage>d__3.MoveNext() +532
System.Linq.Enumerable.Count(IEnumerable`1 source) +212
EPiServer.Forms.Core.Data.Internal.DdsEncryptedPermanentStorage.LoadSubmissionFromStorage(FormIdentity formIden, String[] submissionIds) +156
EPiServer.Forms.Core.Data.FormDataRepository.DeleteUploadFile(ISubmissionStorage submissionStorage, FormIdentity formIden, String submissionId) +180
EPiServer.Forms.Core.Data.FormDataRepository.DeleteSubmissionData(ISubmissionStorage submissionStorage, FormIdentity formIden, String[] submissionIds) +145
EPiServer.Forms.EditView.Controllers.FormsDataStore.DeleteData(ContentReference contentLink, String[] submissionIds) +998
lambda_method(Closure , ControllerBase , Object[] ) +170
System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +228
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +34
System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +38
System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +66
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +41
System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +71
System.Web.Mvc.Async.<>c_DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b_3f() +385
System.Web.Mvc.Async.<>c_DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b_3f() +385
System.Web.Mvc.Async.<>c_DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b_3f() +385
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) +185
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +38
EPiServer.Shell.Services.Rest.RestControllerBase.EndExecute(IAsyncResult asyncResult) +60
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +393
System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +195
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +128

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Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.7.2623.0