Process parameters initialization issue

I have a proces model for updating employee data (not yet finished but it shouldn't matter):

This process starts with interface that looks like this:

It means that update process starts with choosing employee from the database through dropdown field.

I've created these process variables:

The problem is: If I try to run the process for debugging without initializing null values for these parameters I keep getting following error:

If I initialize them to nulls explicitly everything works fine. What could be the reason?

It seems that there is a problem with the initial employee dropdown field which is defined like this:

a!sectionLayout(
          label: "Employee Selection",
          contents: {
            a!dropdownField(
              label: "Employee",
              labelPosition: "ABOVE",
              placeholderLabel: "--- Select a Value ---",
              choiceLabels: a!forEach(
                items: local!employees,
                expression: fv!item.firstName & " " & fv!item.lastName
              ),
              choiceValues: a!forEach(
                items: local!employees,
                expression: fv!item.id
              ),
              value: ri!selectedEmployee,
              saveInto: ri!selectedEmployee,
              required: true,
              validations: {}
            )
          },
          showWhen: local!activeStep = 1
        ),

Local variable used for populating dropdown (local!employees) is defined like this:

a!localVariables(
  local!activeStep: 1,
  local!steps: {
    "Select employee",
    "Personal Data",
    "Address Data",
    "Review"
  },
  local!startAddressId,
  with(
    local!supervisors: rule!EEDM_GetAllSupervisors(),
    local!employees: rule!EEDM_GetAllEmployees(),
    local!allAddress: rule!EEDM_GetAllAddresses(),
    a!formLayout(
      label: "Update Employee"
      
      ...

There should be no problem with EEDM_GetAllEmployees which fetches all employees.