1071654 New Contract from 360Degree: contact couldn't be changed

entity/Contract_entity/entityfields/contact_id/valueProcess.js

 import("system.result");
 import("system.vars");
 
-if (vars.exists("$param.ContactId_param")) 
+if (vars.exists("$param.ContactId_param") && vars.get("$this.value") == null) 
 {
     result.string(vars.get("$param.ContactId_param"));
 }
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