Fix readyPromise condition

Author: ggetz

packages/widgets/Source/Viewer/Viewer.js

@@ -2154,7 +2154,7 @@ function zoomToOrFly(that, zoomTarget, options, isFlight) {
       if (defined(zoomTarget.imageryProvider)) {
         // This is here for backward compatibility. It can be removed when readyPromise is removed.
         let promise = Promise.resolve();
-        if (defined(zoomTarget.imageryProvider)) {
+        if (defined(zoomTarget.imageryProvider._readyPromise)) {
           promise = zoomTarget.imageryProvider._readyPromise;
         } else if (defined(zoomTarget.imageryProvider.readyPromise)) {
           promise = zoomTarget.imageryProvider.readyPromise;