Calendar in editable ALV

Hi,

I have created an Editable ALV using OOP. My requirement is to show calendar for inputting a date on a field.

I have created event handler and register for events.

I have also appended in Fieldcat

lt_fieldcatalog-f4availabl = 'X'.

lt_fieldcatalog-ref_table = 'SYST'.

lt_fieldcatalog-ref_field = 'DATUM'.

But still not working.

I am getting an runtime error "GETWA_NOT_ASSIGNED"

Short text : "Field symbol has not yet been assigned."

Please guide me to implement calendar on F4.

Thanks