Problème with the use of Trigger when service order is posted.

Abdel1978

Hello everyone,
Please I need your help.
I have one CodeUnit that I want to run when I post Service order.
In post action of Page 5900, I call the codeunit (CodeUnitName.RUN(xRec);) but I receive the following message.
.
I think that the error occures because when I post "Service order" the related data was submitted.
Which trigger should I use to run properly the codeunit?

I work with Microsoft Dynamcs 2016
the codeunit look like:
*****************
OnRun(VAR Rec : Record "Service Header")
BEGIN

//Objects initializations
StringBuilder := StringBuilder.StringBuilder;
StringWriter := StringWriter.StringWriter(StringBuilder);
JSONTextWriter := JSONTextWriter.JsonTextWriter(StringWriter);

//Starting the JSON serialization
JSONTextWriter.WriteStartObject;


ServiceHeader.GET("Document Type"::Order,"No.");
CreateJSONAttribute('EntId',5);
CreateJSONAttribute('EntKey','TEST3D');
CreateJSONAttribute('DMSId',3);
CreateJSONAttribute('RdvId',RdvId);
CreateJSONAttribute('RdvRONum',"No.");
CreateJSONAttribute('RdvState',2);