Having issue with making a script I had written works okay

Dear All,

I am working on a form, that requires the Signature fields to be "readOnly", unless the two fields fields before it are completed. If the fields are not completed it should show a message, but if these fields are completed no message should show.

I wrote the following script and put on the on enter event of the Signature field, but it does not work well.

var v;

if(form1.Page1.Reviewerssignat.rawValue ==null){

  Reviewerssignat.access="readOnly";

  v=xfa.host.messageBox("Please Complete Supervisor  and Senior Supervisor fields");

}

else if(form1.Page1.Reviewerssignat.rawValue !=null){  

  Reviewerssignat.access="open";

  v=xfa.host.messageBox("");

}

Please can someone out there tell me why it is not working.

Thanks

Tammy