move attribution from constructor to requestImage

Author: lukemckinstry

packages/engine/Source/Scene/Google2DImageryProvider.js

@@ -124,8 +124,6 @@ function Google2DImageryProvider(options) {
   // This will be defined for ion resources
   this._tileCredits = resource.credits;
   this._attributionsByLevel = undefined;
-  // Asynchronously request and populate _attributionsByLevel
-  this.getViewportCredits();
 }
 
 Object.defineProperties(Google2DImageryProvider.prototype, {
@@ -485,7 +483,21 @@ Google2DImageryProvider.prototype.requestImage = function (
   level,
   request,
 ) {
-  return this._imageryProvider.requestImage(x, y, level, request);
+  const promise = this._imageryProvider.requestImage(x, y, level, request);
+
+  // If the requestImage call returns undefined, it couldn't be scheduled this frame. Make sure to return undefined so this can be handled upstream.
+  if (!defined(promise)) {
+    return undefined;
+  }
+
+  // Asynchronously request and populate _attributionsByLevel if it hasn't been already. We do this here so that the promise can be properly awaited.
+  if (promise && !defined(this._attributionsByLevel)) {
+    return Promise.all([promise, this.getViewportCredits()]).then(
+      (results) => results[0],
+    );
+  }
+
+  return promise;
 };
 
 /**