Umbraco custom workflow throws "Object reference not set to an instance of an object" but is never hit?

We had a custom workflow that was working until we upgraded to Umbraco Forms 7.4.1. Since then, the custom workflow throws the following error

[NullReferenceException: Object reference not set to an instance of an object.]
    Umbraco.Forms.Data.StringHelper.ParseRecordPlaceholders(String value, IRecord record) +237
    Umbraco.Forms.Data.StringHelper.ParsePlaceHolders(HttpContext context, Record record, String value) +74
    Umbraco.Forms.Data.StringHelper.ParsePlaceHolders(Record record, String value) +39
    Umbraco.Forms.Core.Services.WorkflowService.ExecuteWorkflows(List`1 workflows, RecordEventArgs e) +453
    Umbraco.Forms.Core.Services.WorkflowService.ExecuteWorkflows(Record record, Form form, FormState state, Boolean editMode) +183
    Umbraco.Forms.Web.Services.RecordService.Approve(Record record, Form form) +287
    Umbraco.Forms.Web.Services.RecordService.Submit(Record record, Form form) +702
    Umbraco.Forms.Web.Controllers.UmbracoFormsController.SubmitForm(Form form, FormViewModel model, Dictionary`2 state, ControllerContext context) +1947
    Umbraco.Forms.Web.Controllers.UmbracoFormsController.GoForward(Form form, FormViewModel model, Dictionary`2 state) +58
    Umbraco.Forms.Web.Controllers.UmbracoFormsController.HandleForm(FormViewModel model, Boolean captchaIsValid) +442
    lambda_method(Closure , ControllerBase , Object[] ) +143
    System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +14
    System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +157
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +37
    System.Web.Mvc.Async.<>c.<BeginInvokeSynchronousActionMethod>b__9_0(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +22
    System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +29
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +32
    System.Web.Mvc.Async.<>c__DisplayClass11_0.<InvokeActionMethodFilterAsynchronouslyRecursive>b__0() +58
    System.Web.Mvc.Async.<>c__DisplayClass11_2.<InvokeActionMethodFilterAsynchronouslyRecursive>b__2() +228
    System.Web.Mvc.Async.<>c__DisplayClass11_2.<InvokeActionMethodFilterAsynchronouslyRecursive>b__2() +228
    System.Web.Mvc.Async.<>c__DisplayClass11_2.<InvokeActionMethodFilterAsynchronouslyRecursive>b__2() +228
    System.Web.Mvc.Async.<>c__DisplayClass11_2.<InvokeActionMethodFilterAsynchronouslyRecursive>b__2() +228
    System.Web.Mvc.Async.<>c__DisplayClass11_2.<InvokeActionMethodFilterAsynchronouslyRecursive>b__2() +228
    System.Web.Mvc.Async.<>c__DisplayClass7_0.<BeginInvokeActionMethodWithFilters>b__1(IAsyncResult asyncResult) +10
    System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +34
    System.Web.Mvc.Async.<>c__DisplayClass3_6.<BeginInvokeAction>b__4() +35
    System.Web.Mvc.Async.<>c__DisplayClass3_1.<BeginInvokeAction>b__1(IAsyncResult asyncResult) +100
    System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +27
    System.Web.Mvc.<>c.<BeginExecuteCore>b__152_1(IAsyncResult asyncResult, ExecuteCoreState innerState) +11
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +29
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
    System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +45
    System.Web.Mvc.<>c.<BeginExecute>b__151_2(IAsyncResult asyncResult, Controller controller) +13
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +22
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
    System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +26
    System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10
    System.Web.Mvc.<>c.<BeginProcessRequest>b__20_1(IAsyncResult asyncResult, ProcessRequestState innerState) +28
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +29
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
    System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +28
    System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
    System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +584
    System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +132
    System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +173

When the workflow is disabled, the form submits and redirects as expected.

When attempting to debug the issue locally, the error is being thrown before the custom workflow is being hit suggesting there maybe an underlying issue somewhere else.

I can see there there may have been a fix in version 8.5.1 (https://our.umbraco.com/packages/developer-tools/umbraco-forms/) but could the same issue be affecting version 7.4.1.

Any ideas?