handle undefined/deferred return

packages/engine/Source/Core/CesiumTerrainProvider.js

@@ -897,11 +897,29 @@ CesiumTerrainProvider.prototype.requestTileGeometry = function (
 
   if (!defined(layerToUse) && unknownAvailability) {
     // Try again when availability data is ready– Otherwise the tile will be marked as failed and never re-requested
-    return availabilityPromise.then(() =>
-      this.requestTileGeometry(x, y, level, request)
-    );
+    return availabilityPromise.then(() => {
+      // handle promise or undefined return
+      return new Promise((resolve, reject) => {
+        // defer execution to the next event loop
+        setTimeout(() => {
+          const originalResult = this.requestTileGeometry(x, y, level, request);
+          if (originalResult && typeof originalResult.then === "function") {
+            originalResult
+              .then((result) => {
+                resolve(result);
+              })
+              .catch((err) => {
+                reject(err);
+              });
+          } else {
+            // undefined means deferred, wait for the promise to resolve
+            return resolve(this.requestTileGeometry(x, y, level, request));
+          }
+        }, 0); // next tick
+      });
+    });
   }
-
+  // call overridden function below
   return requestTileGeometry(this, x, y, level, layerToUse, request);
 };