fix build problem

src/core/scene/a-scene.js

@@ -278,7 +278,7 @@ class AScene extends AEntity {
     var xrInit;
 
     // Don't enter VR if already in VR.
-    if (useOfferSession && navigator.xr.offerSession === undefined) { return Promise.resolve('OfferSession is not supported.'); }
+    if (useOfferSession && (navigator.xr === undefined || navigator.xr.offerSession === undefined)) { return Promise.resolve('OfferSession is not supported.'); }
     if (self.usedOfferSession && useOfferSession) { return Promise.resolve('OfferSession was already called.'); }
     if (this.is('vr-mode')) { return Promise.resolve('Already in VR.'); }