title: How to fix 'TypeError: this.isCallback is not a function' error for UserAgentApplication in msal.js
body: <p>using Msal v1.0.2, loginPopup is not working from iFrame.</p>
  
  <p>trying to get the UserAgentApplication instance using client_id. its throwing an exception:
  TypeError: this.isCallback is not a function
      at Object.UserAgentApplication (UserAgentApplication.ts:228)</p>
  
  <pre><code>const myMSALObj = Msal.UserAgentApplication(msalConfig);
  myMSALObj.loginPopup(["user.read"]).then(function (loginResponse) {
            return myMSALObj.acquireTokenSilent(accessTokenRequest);
          }).then(function (accessTokenResponse) {
            const token = accessTokenResponse.accessToken;
          }).catch(function (error) {
            //handle error
          });
  </code></pre>
  
  <p>sample from . 'Quickstart for MSAL JS' works fine but when I try to integrate Msal inside iFrame of my JavaScript plugin code, its not working.</p>
  
  <h2>working code from sample:</h2>
  
  <pre><code>var myMSALObj = new Msal.UserAgentApplication(msalConfig);
  myMSALObj.handleRedirectCallback(authRedirectCallBack);
  
  myMSALObj.loginPopup(requestObj).then(function (loginResponse) {
  acquireTokenPopupAndCallMSGraph();
      }).catch(function (error) {
          //Please check the console for errors
          console.log(error);
      });
  </code></pre>