[WIP] LocalWorkflowClient.CreateAndRunInstanceAsync function raises System.NullReferenceException

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

We have ASP.NET Core Web API application which is based on .NET 8. This application uses Elsa Workflows NuGets 3.3.0-rc7. Everything works well with this configuration. But... when we update NuGets, we get System.NullReferenceException from LocalWorkflowClient.CreateAndRunInstanceAsync function. We tried all versions up to 3.3.5 and got the same exception.

I will try to describe what exactly we do. We create a root activity which in turn includes other activities as a children. All these activities are creating by our own code. We don't use any builders from Elsa Workflows NuGets. Then, we save Elsa Workflows definition to a database as you can see it below on the figure.

If Elsa Workflows definition has been saved in a database successfully, we try to create and run a workflow.

The exception is raising here.

After my investigation, I found out that this private field doesn't have any items.

Due to it, functionality can't find any ActivityDescriptor during workflow instantiation.

Everything works well when I add this code snippet.

Because, after that, _activityDescriptors private field contains items. The field will be filled in too if you open any workflow definitions in Elsa Workflows Dashboard firstly.

The stack trace is...

at Elsa.Workflows.ActivityFactory.ReadSyntheticInputs(ActivityDescriptor activityDescriptor, IActivity activity, JsonElement activityRoot, JsonSerializerOptions options)
at Elsa.Workflows.ActivityFactory.Create(Type type, ActivityConstructorContext context)
at Elsa.Workflows.Serialization.Converters.ActivityJsonConverter.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options)
at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value, Boolean& isPopulatedValue)
at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 utf8Json, JsonTypeInfo`1 jsonTypeInfo, Nullable`1 actualByteCount)
at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo`1 jsonTypeInfo)
at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
at Elsa.Workflows.Serialization.Serializers.JsonActivitySerializer.Deserialize(String serializedActivity)
at Elsa.Workflows.Management.Mappers.WorkflowDefinitionMapper.Map(WorkflowDefinition source)
at Elsa.Workflows.Management.Materializers.JsonWorkflowMaterializer.ToWorkflow(WorkflowDefinition definition)
at Elsa.Workflows.Management.Materializers.JsonWorkflowMaterializer.MaterializeAsync(WorkflowDefinition definition, CancellationToken cancellationToken)
at Elsa.Workflows.Management.Services.WorkflowDefinitionService.MaterializeWorkflowAsync(WorkflowDefinition definition, CancellationToken cancellationToken)
at Elsa.Workflows.Management.Services.WorkflowDefinitionService.FindWorkflowGraphAsync(WorkflowDefinitionFilter filter, CancellationToken cancellationToken)
at Elsa.Workflows.Management.Services.WorkflowDefinitionService.FindWorkflowGraphAsync(WorkflowDefinitionHandle definitionHandle, CancellationToken cancellationToken)
at Elsa.Workflows.Runtime.LocalWorkflowClient.GetWorkflowGraphAsync(WorkflowDefinitionHandle definitionHandle, CancellationToken cancellationToken)
at Elsa.Workflows.Runtime.LocalWorkflowClient.CreateInstanceInternalAsync(CreateWorkflowInstanceRequest request, CancellationToken cancellationToken)
at Elsa.Workflows.Runtime.LocalWorkflowClient.CreateAndRunInstanceAsync(CreateAndRunWorkflowInstanceRequest request, CancellationToken cancellationToken)

Could you please tell me what we are doing wrong or if it is a bug?

Fixes #6594.

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