Setting null to variable

Hello!

I need to set different pairs of fields, depending on the user's choice. If the field is not necessary to set, there should be empty. Fields are lookup fields with ID's value. BIGINT type, maybe? I try so:

tw = if(input.tBotcl, referencies.states.getIdByEng("onClientHands"), referencies.states.getIdByEng("employee"));

te = if(input.tBotcl, null, input.employee);

tcl = if(input.tBotcl, input.client, null);

insertConfig = insert into changeConfig

[

...

tBwhere = tw

tBEmployee = te

tBClient = tcl

...

];

But Zoho say: "Both the argument should be same type for 'IF' task. Expecting 'IF' expression found 'NULLTYPE' expression"

I read in https://help.zoho.com/portal/en/community/topic/set-null-variable that "assign null value to all type of fields".

What I'm doing wrong? How do I clean up a variable to write an empty value?

Thanks, Ulia