passing a variable in a stored procedure

I have a working stored procedure as seen below

 Declare
      l_id number;
    begin
      l_id := apex_mail.send( 
        p_to => 'test@test.com', 
        p_from => 'test@test.com', 
        p_subj => 'Mail from APEX with attachment', 
        p_body => 'Please review the attachment.', 
        p_body_html => 'Please review the attachment.'
      );
      apex_mail.add_attachment( 
         p_mail_id    => l_id,
        p_attachment => p_output_blob, 
        p_filename   => p_output_filename, 
        p_mime_type  => p_output_mime_type
      );
     end;

This procedure emails an attachment after it is downloaded, which is what I want. This issue however is, I will like the p_to to change based on the new email_address in my table called TEMP_FEES. The table will have one record/email address at a time.

I have tried

Declare
  l_id number;
begin
    FOR m IN (SELECT parent_email
            FROM TEMP_FEES
           )
LOOP
  apex_mail.send( p_to => m.parent_email,
                  p_from => 'test@test.com',
                  p_body => 'test',
                  p_subj => 'invoice'
  );
  apex_mail.add_attachment( 
    p_mail_id    => l_id, 
    p_attachment => p_output_blob, 
    p_filename   => p_output_filename, 
    p_mime_type  => p_output_mime_type
  );
 
  END LOOP;  
end

But I am getting an error

ORA-20022: Null value supplied for parameter P_mail_id

when I submit the form.

Can I get any suggestions on how to solve this?

Not sure if this could be the issue, but you are not assigning the procedure call output to the variable l_id in your second example. So the code should be as follows:-

Declare
  l_id number;
begin
    FOR m IN (SELECT parent_email
            FROM TEMP_FEES
           )
LOOP
  l_id := apex_mail.send( p_to => m.parent_email,
                  p_from => 'test@test.com',
                  p_body => 'test',
                  p_subj => 'invoice'
  );
  apex_mail.add_attachment( 
    p_mail_id    => l_id, 
    p_attachment => p_output_blob, 
    p_filename   => p_output_filename, 
    p_mime_type  => p_output_mime_type
  );
 
  END LOOP;  
end;

Hope this helps.

Thanks Arnab P