How to inject the variable in the abstract class while unit testing the subclass?

I have an abstract class BaseTemplate and multiple classes extending it. In one of the concrete class(SmsTemplate extends BaseTemplate), we have a private variable Gson. We have the same private variable(Gson) in the abstract class as well.

While unit tesing the concrete class, methods in the abstract class is getting called from the concrete class. In my Unit test, I am using Whitebox.setInternalState(smsTemplateObj, gsonObj); to inject the Gson object into the private members of SmsTemplate and BaseTemplate but the Gson is getting injected only in the subclass. In abstract class, its NULL, meaning not injected. Below is the implementation.

Can someone please tell how to inject the Gson object in the abstract class?

abstract class BaseTemplate{

    private Gson gson;//Here its not getting injected

    protected String getContent(Content content){
        return gson.toJson(content); // ERROR - gson here throws NPE as its not injected
    }
}

class SmsTemplate extends BaseTemplate{

    private Gson gson;//Here its getting injected

    public String processTemplate(Content content){
        String strContent = getContent(content);
        ...
        ...
        gson.fromJson(strContent, Template.class);
    }
}