Object reference not set to an instance of an object.

[Yuval2612]

Describe the bug
I'm using WorkFlowCore with asp.net core, at the end of each workflow I see in the log 'fail: WorkflowCore.Services.BackgroundTasks.WorkflowConsumer[0]'
I look into it and saw that somehow the function GetWorkflowInstance was called and the workflows are empty.
this function is located in the class that uses IPersistenceProvider.

To Reproduce
Create a flow from JSON

Expected behavior
The null exception should not appear.

Additional context
example of the error log:
fail: WorkflowCore.Services.BackgroundTasks.WorkflowConsumer[0]
Error executing item 41b5ef64-ba42-4511-8ccc-7b78d65adc80 - Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.
at WorkflowCore.Services.BackgroundTasks.WorkflowConsumer.ProcessItem(String itemId, CancellationToken cancellationToken)
at WorkflowCore.Services.BackgroundTasks.WorkflowConsumer.ProcessItem(String itemId, CancellationToken cancellationToken)
at WorkflowCore.Services.BackgroundTasks.QueueConsumer.ExecuteItem(String itemId, EventWaitHandle waitHandle, Activity activity)

this is the JSON flow
{
"Id": "ny",
"Version": 1,
"DataType": "Getb.Evaluation.Flow.Manager.FlowExecution.FlowContext, Getb.Evaluation.Flow.Manager",
"ErrorBehavior":"Terminate",
"Steps": [
{
"Id": "Initialization",
"StepType": "Getb.Evaluation.Flow.Manager.Steps.Initialization, Getb.Evaluation.Flow.Manager",
"ErrorBehavior":"Terminate",
"NextStepId": "ParallelTracking"
},
{
"Id": "ParallelTracking",
"StepType": "WorkflowCore.Primitives.Sequence, WorkflowCore",
"ErrorBehavior":"Terminate",
"NextStepId": "Evaluation",
"Do": [
[
{
"Id": "Tracking",
"StepType": "Getb.Evaluation.Flow.Manager.Steps.Tracking, Getb.Evaluation.Flow.Manager",
"ErrorBehavior":"Terminate",
"Inputs": {
"System": ""ny"",
"IgnorePlots": ""py""
}
}
]
]
},
{
"Id": "Evaluation",
"StepType": "Retb.Evaluation.Flow.Manager.Steps.Evaluation, Retb.Evaluation.Flow.Manager",
"ErrorBehavior":"Terminate",
"NextStepId": "Finalization",
"Inputs": {
"System": ""ny""
}
},
{
"Id": "Finalization",
"StepType": "Retb.Evaluation.Flow.Manager.Steps.Finalization, Retb.Evaluation.Flow.Manager",
"ErrorBehavior":"Terminate"
}
]
}

[scccboom]

This is usually caused by incorrect persistence, and sometimes the workflow ends after it is started.
You should provide more detailed information. For example persistence code, database

[eli-ilushin]

I've encountered a similar issue and it was caused because in some cases my implementation of IPersistenceProvider was returning null from GetWorkflowInstance(...)