Fixed SkyMaterial. Raised an error when cached defines were equal to the current defines

materialsLibrary/dist/babylon.skyMaterial.js

@@ -90,7 +90,7 @@ var BABYLON;
                 }
             }
             // Get correct effect      
-            if (!this._defines.isEqual(this._cachedDefines)) {
+            if (!this._defines.isEqual(this._cachedDefines) || !this._effect) {
                 this._defines.cloneTo(this._cachedDefines);
                 scene.resetCachedMaterial();
                 // Fallbacks

materialsLibrary/materials/sky/babylon.skyMaterial.ts

@@ -110,7 +110,7 @@ module BABYLON {
             }
 
             // Get correct effect      
-            if (!this._defines.isEqual(this._cachedDefines)) {
+            if (!this._defines.isEqual(this._cachedDefines) || !this._effect) {
                 this._defines.cloneTo(this._cachedDefines);
                 
                 scene.resetCachedMaterial();