I am trying to send information to a dialog box in an Outlook web-add in.

var url = window.location.origin+'/dialog.html'
var dialog
Office.context.ui.displayDialogAsync(url2,
       function (asyncResult) {
           dialog = asyncResult.value;
           dialog.messageChild('message')     
           });

Which opens a dialog box and tries to send information to the dialog which has this Javascript:

Office.onReady().then(()=> {
        Office.context.ui.addHandlerAsync(
            Office.EventType.DialogParentMessageReceived,
            onMessageFromParent);         
    });

function onMessageFromParent(arg){
    console.log(arg.message)
    document.getElementById('ID').style.display = 'none';
}

However, that just gives me the error:

TypeError: Office.context.ui.addHandlerAsync is not a function

When I run console.log(Office.context.ui), I get:

OSF.DDA.UI.ParentUI
[[Prototype]]:Object
closeContainer:function(){ … }
displayDialogAsync:function(){ … }
openBrowserWindow:function(){ … }
__proto__:Object

As you can see, there is not a addHandlerAync function within Office.context.ui but the Microsoft documentation says to use it.

What is going wrong here? Thanks