WhoID is null when send email in sandbox on Contacts

I have created a trigger on task (before Insert) and just displaying the trigger.new value ,i.e, the task which is created . In the debug it is showing me that whoid is null , I am sending the email from "Send An Email" button on the activity history of contacts .

Here is the code for the trigger :-

trigger triggerOnTask on Task (before insert) {
if(trigger.isBefore && trigger.isInsert){
for(task ts : trigger.new){
System.debug('>>>>>>ts'+ts);
System.debug('>>>>>>'+ts.Description);
System.debug('>>>>>>'+ts.whoId);
Id myId=ts.whoId;
}
}
}

In the above code, whoId is null in debug when I click on send Email button.

Any suggestions why is the WhoId null in the task created.

Also , this is working in my dev org and I am getting the whoid value but in sandbox it is working on leads but not working on the Contacts.