Fix exception when root imagery tiles fail to load.

Source/Scene/CentralBodySurface.js

@@ -670,7 +670,7 @@ define([
                     if (imageryLayer.getImageryProvider().isReady()) {
                         // Remove the placeholder and add the actual skeletons (if any)
                         // at the same position.  Then continue the loop at the same index.
-                        imagery.releaseReference();
+                        tileImagery.freeResources();
                         tileImageryCollection.splice(i, 1);
                         imageryLayer._createTileImagerySkeletons(tile, terrainProvider, i);
                         --i;
@@ -704,6 +704,15 @@ define([
                         parent = parent.parent;
                     }
 
+                    // If there's no valid parent, remove this TileImagery from the tile.
+                    if (typeof parent === 'undefined') {
+                        tileImagery.freeResources();
+                        tileImageryCollection.splice(i, 1);
+                        --i;
+                        len = tileImageryCollection.length;
+                        continue;
+                    }
+
                     // use that parent imagery instead, storing the original imagery
                     // in originalImagery to keep it alive
                     tileImagery.originalImagery = imagery;

Specs/Scene/CentralBodySurfaceSpec.js

@@ -17,7 +17,8 @@ defineSuite([
          'Scene/ImageryLayerCollection',
          'Scene/OrthographicFrustum',
          'Scene/SceneMode',
-         'Scene/SingleTileImageryProvider'
+         'Scene/SingleTileImageryProvider',
+         'Scene/WebMapServiceImageryProvider'
      ], function(
          CentralBodySurface,
          createContext,
@@ -36,7 +37,8 @@ defineSuite([
          ImageryLayerCollection,
          OrthographicFrustum,
          SceneMode,
-         SingleTileImageryProvider) {
+         SingleTileImageryProvider,
+         WebMapServiceImageryProvider) {
     "use strict";
     /*global jasmine,describe,xdescribe,it,xit,expect,beforeEach,afterEach,beforeAll,afterAll,spyOn,runs,waits,waitsFor*/
 
@@ -346,5 +348,25 @@ defineSuite([
                 expect(render(context, frameState, cb)).toBeGreaterThan(0);
             });
         });
+
+        it('renders even if imagery root tiles fail to load', function() {
+            var layerCollection = cb.getImageryLayers();
+            layerCollection.removeAll();
+
+            var providerWithInvalidRootTiles = new WebMapServiceImageryProvider({
+                url : '/invalid',
+                layers : 'invalid'
+            });
+
+            layerCollection.addImageryProvider(providerWithInvalidRootTiles);
+
+            frameState.camera.viewExtent(new Extent(0.0001, 0.0001, 0.0025, 0.0025), Ellipsoid.WGS84);
+
+            updateUntilDone(cb);
+
+            runs(function() {
+                expect(render(context, frameState, cb)).toBeGreaterThan(0);
+            });
+        });
     });
 });