NRE when creating a new workflow in a fresh new installation

When creating a new workflow from scratch in a fresh new database the actvitiyRegistry is empty and the activityTypeName is "Elsa.Flowchart" instead of "NotFoundActivity"

```
// If the activity type is a NotFoundActivity, try to extract the original activity type name and version.
if (activityTypeName.Equals(notFoundActivityTypeName) && activityRoot.TryGetProperty("originalActivityJson", out var originalActivityJson))
{
activityRoot = JsonDocument.Parse(originalActivityJson.GetString()!).RootElement;
activityTypeName = GetActivityDetails(activityRoot, out activityTypeVersion, out activityDescriptor);
}

```
So
`var notFoundActivityDescriptor = activityRegistry.Find()!; `
is null in
`elsa-core\src\modules\Elsa.Workflows.Core\Serialization\Converters\ActivityJsonConverter.cs`

this lead to a NRE in both methods below

```
private void ReadSyntheticInputs(ActivityDescriptor activityDescriptor, IActivity activity, JsonElement activityRoot, JsonSerializerOptions options)

private void ReadSyntheticOutputs(ActivityDescriptor activityDescriptor, IActivity activity, JsonElement activityRoot)
and

```
in
`elsa-core\src\modules\Elsa.Workflows.Core\Services\ActivityFactory.cs`

**and this lead to a not working and unusable "Elsa Server" instance**

```
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.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader)
at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
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.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader)
at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
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, T& value, JsonSerializerOptions options, ReadStack& state)
at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.ContinueDeserialize(ReadBufferState& bufferState, JsonReaderState& jsonReaderState, ReadStack& readStack, T& value)
at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.DeserializeAsync(Stream utf8Json, CancellationToken cancellationToken)
at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.DeserializeAsObjectAsync(Stream utf8Json, CancellationToken cancellationToken)
at FastEndpoints.RequestBinder`1.BindJsonBody(HttpRequest httpRequest, JsonSerializerContext serializerCtx, CancellationToken ct)
at FastEndpoints.RequestBinder`1.BindAsync(BinderContext ctx, CancellationToken ct)
at FastEndpoints.Endpoint`2.BindRequestAsync(EndpointDefinition def, HttpContext ctx, List`1 failures, CancellationToken ct)
at FastEndpoints.Endpoint`2.ExecAsync(CancellationToken ct)
at FastEndpoints.Endpoint`2.ExecAsync(CancellationToken ct)
at Elsa.Http.Middleware.HttpWorkflowsMiddleware.InvokeAsync(HttpContext httpContext, IServiceProvider serviceProvider)
at Microsoft.AspNetCore.Localization.RequestLocalizationMiddleware.Invoke(HttpContext context)
at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddlewareImpl.g__Awaited|10_0(ExceptionHandlerMiddlewareImpl middleware, HttpContext context, Task task)

```