Fix crash with switching terrain provider and picking

CHANGES.md

@@ -23,6 +23,7 @@ Change Log
 * Improved `Plane` entities so they are better aligned with the globe surface [#6887](https://github.com/AnalyticalGraphicsInc/cesium/pull/6887)
 * Fixed crash when rendering translucent objects when all shadow maps in the scene set `fromLightSource` to false. [#6883](https://github.com/AnalyticalGraphicsInc/cesium/pull/6883)
 * Fixed night shading in 2D and Columbus view. [#4122](https://github.com/AnalyticalGraphicsInc/cesium/issues/4122)
+* Fixed crash that happened when calling `scene.pick` after setting a new terrain provider [#6918](https://github.com/AnalyticalGraphicsInc/cesium/pull/6918)
 
 ### 1.48 - 2018-08-01
 

Source/Scene/QuadtreePrimitive.js

@@ -300,6 +300,11 @@ define([
             return;
         }
 
+        if (this._tilesInvalidated) {
+            invalidateAllTiles(this);
+            this._tilesInvalidated = false;
+        }
+
         // Gets commands for any texture re-projections
         this._tileProvider.initialize(frameState);
 
@@ -376,16 +381,10 @@ define([
             return;
         }
 
-        if (this._tilesInvalidated) {
-            invalidateAllTiles(this);
-        }
-
         // Load/create resources for terrain and imagery. Prepare texture re-projections for the next frame.
         processTileLoadQueue(this, frameState);
         updateHeights(this, frameState);
         updateTileLoadProgress(this, frameState);
-
-        this._tilesInvalidated = false;
     };
 
     /**

Specs/Scene/GlobeSurfaceTileProviderSpec.js

@@ -674,12 +674,14 @@ defineSuite([
                 var surface = scene.globe._surface;
                 var replacementQueue = surface._tileReplacementQueue;
                 expect(replacementQueue.count).toBeGreaterThan(0);
+                var oldTile = replacementQueue.head;
 
                 surface.tileProvider.terrainProvider = new EllipsoidTerrainProvider();
 
                 scene.renderForSpecs();
 
-                expect(replacementQueue.count).toBe(0);
+                expect(replacementQueue.count).toBeGreaterThan(0);
+                expect(replacementQueue.head).not.toBe(oldTile);
             });
         });