'not_specified' error while calling Office.context.mailbox.item.body.getTypeAsync

Question

Why am I getting not_specified error sometimes while getting Outlook item body type? I'm checking that Office.context.mailbox.item exists before calling this method:

sfMailApp.OfficeManager.prototype._getTypeAsync = function () {
var deferred = $.Deferred();
var _this = this;

var item = this.getItem();

if (this._isComposeMode && item.body && !item.isFake) {
    var currentBodyType = this.bodyType;
    item.body.getTypeAsync(function (asyncResult) {
        if (!sfMailApp.utils.isNullOrEmpty(asyncResult) && !sfMailApp.utils.isNullOrEmpty(asyncResult.value) && asyncResult.status === Office.AsyncResultStatus.Succeeded) {
            _this.bodyType = asyncResult.value.data ? asyncResult.value.data : asyncResult.value;
            if (currentBodyType !== _this.bodyType) {
                _this.events.onBodyTypeChanged._fire();
            }
            deferred.resolve(_this.bodyType);
        }
        else {
            _this._onError(asyncResult);
            deferred.reject();
        }
    });
} else {
    deferred.resolve(_this.bodyType);
}

return deferred.promise();};

Call stack shows following:

Error
at n.prototype.execute (appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:11:86986)
at u.DDA.OutlookAppOm.prototype.callOutlookDispatcher (appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:11:152689)
at u.DDA.OutlookAppOm.prototype.invokeHostMethod (appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:11:152558)
at u.DDA.OutlookAppOm.prototype._standardInvokeHostMethod$i$0 (appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:11:150832)
at r.ComposeBody.prototype.getTypeAsync (appsforoffice.microsoft.com/lib/1.1/hosted/outlook-win32-16.02.js:11:182704)
at sfMailApp.OfficeManager.prototype._getTypeAsync (smart-addinsf.invisible.io/Scripts/minified/main.js?v=1503941093:30:11571)
at sfMailApp.OfficeManager.prototype.reloadItemInfo (smart-addinsf.invisible.io/Scripts/minified/main.js?v=1503941093:30:28930)
at sfMailApp.OfficeManager.prototype._reloadAndAnalyzeItemInfoChanges (smart-addinsf.invisible.io/Scripts/minified/main.js?v=1503941093:30:16965)