Problem with the null in conditional column

Hi everbody, 

I have the problem with conditional column through query editor. I add column by conditions and all except null. The values of rows to which appeal conditions, sometimes equals to null. Added condtional column contains required results but when the condition faces null then i receive an error. How can i replace this? I've tried condition if null then -, but it's not worked

Expression.Error: We cannot convert the value null to type Logical.
Details:
Value=
Type=Type