How to use CLOB in decode??

I have a very simple table as below:

CREATE TABLE C_POST

(

  POST_ID               INTEGER                 NOT NULL,

  POST_REPLY_SET        CLOB

)

Now I just want to get this field "POST_REPLY_SET" as this format:

If the field "POST_REPLY_SET" is NULL, it should be changed to string "NULL" in my query result.

If the field "POST_REPLY_SET" is not NULL, it should be dbms_lob.substr(post_reply_set).

And based on the string of "POST_REPLY_SET", if it contains change line and return, I need replace change line and return to a space " ".

How to write this SQL? I tried this one: