SharePoint List Get List Item Reference error "Object reference not set to an instance of an object"

On SharePoint, I have 2 list: MyWorkflowList and MyList.
MyList has a column "RoleName", and the values are :

-Admin

-Approver

MyWorkflowList is a Sub Workflow, called manually from another workflow. MyWorkflowList has a "Get List Item Reference".
On the "Specify the list containing the list item to reference", I select "MyList".
On the "Specify filters for creating the reference", I select "RoleName" with value = Approver

I set the "Reference Name" to be "Events Approver"

During execution/runtime when it gets to this stage, I am getting an error "Object reference not set to an instance of an object"

How can I fix this error ?

Thank you.