Security, SSL Pinning in Mobile

I’ve found two solutions for SSL pinning in ionic mobile apps (https://github.com/EddyVerbruggen/SSLCertificateChecker-PhoneGap-Plugin and https://github.com/wymsee/cordova-HTTP) and not much else in terms of people writing ionic apps actually using them. I’m trying the above Eddy plugin in ionic 2. Base functionality works. When I add my code to the functions, I get the below error and not sure what it means. Anyone have experience with this plugin and/or know what I’m doing wrong?

Error:
Error in Error callbackId: SSLCertificateChecker1359693488 : TypeError: null is not an object (evaluating 'this.getFriends’)

My Code being called in constructor:

checker(){
    window["plugins"].sslCertificateChecker.check(
                successCallback,
                errorCallback,
                this.server,
                this.fingerprint);
     function successCallback(message) {
       // Message is always: CONNECTION_SECURE.
       console.log("cert success");
       // Now do something with the trusted server.
       this.getFriends(); //https get call
     }

     function errorCallback(message) {
         console.log("cert fail");
         this.showErrorToast(); //show toast with bad news
      }
  
 }

}