test: `DialogStepsFeature.AssignToUserOnAssignDialog`

The test `Capgemini.PowerApps.SpecFlowBindings.UiTests.DialogStepsFeature.AssignToUserOnAssignDialog` is failing.

```
Test method Capgemini.PowerApps.SpecFlowBindings.UiTests.DialogStepsFeature.AssignToUserOnAssignDialog threw exception:
System.ArgumentNullException: input cannot be null
Parameter name: input

at Microsoft.Dynamics365.UIAutomation.Browser.BrowserCommand`1.Execute[T1,T2,T3,T4,T5,T6,T7,T8,T9](IWebDriver driver, T1 p1, T2 p2, T3 p3, T4 p4, T5 p5, T6 p6, T7 p7, T8 p8, T9 p9)
at Microsoft.Dynamics365.UIAutomation.Browser.BrowserCommand`1.Execute(IWebDriver driver)
at Microsoft.Dynamics365.UIAutomation.Browser.BrowserPage.Execute[TResult](BrowserCommandOptions options, Func`2 delegate)
at Capgemini.PowerApps.SpecFlowBindings.Steps.DialogSteps.WhenIAssignToANamedOnTheAssignDialog(AssignTo assignTo, String userName) in \bindings\src\Capgemini.PowerApps.SpecFlowBindings\Steps\DialogSteps.cs:line 41
at TechTalk.SpecFlow.Bindings.BindingInvoker.InvokeBinding(IBinding binding, IContextManager contextManager, Object[] arguments, ITestTracer testTracer, TimeSpan& duration)
at TechTalk.SpecFlow.Infrastructure.TestExecutionEngine.ExecuteStepMatch(BindingMatch match, Object[] arguments)
at TechTalk.SpecFlow.Infrastructure.TestExecutionEngine.ExecuteStep(IContextManager contextManager, StepInstance stepInstance)
at TechTalk.SpecFlow.Infrastructure.TestExecutionEngine.OnAfterLastStep()
```

https://dev.azure.com/capgeminiuk/GitHub%20Support/_build/results?buildId=8397&view=ms.vss-test-web.build-test-results-tab&runId=1015498&resultId=100161&paneView=debug