NPE screenParams before setting rootBackgroundImageName

android/app/src/main/java/com/reactnativenavigation/controllers/NavigationActivity.java

         if (hasBackgroundColor()) {
             layout.asView().setBackgroundColor(AppStyle.appStyle.screenBackgroundColor.getColor());
         }
-        String rootBackgroundImageName = activityParams.screenParams.styleParams.rootBackgroundImageName;
+        String rootBackgroundImageName = getRootBackgroundImageName();
         if (rootBackgroundImageName != null) {
             layout.asView().setBackgroundResource(this.getResources().getIdentifier(rootBackgroundImageName, "drawable", this.getPackageName()));
         }
         setContentView(layout.asView());
     }
 
+    private String getRootBackgroundImageName() {
+        return activityParams.screenParams == null ? null : activityParams.screenParams.styleParams.rootBackgroundImageName;
+    }
+
     private boolean hasBackgroundColor() {
         return AppStyle.appStyle.screenBackgroundColor != null &&
                 AppStyle.appStyle.screenBackgroundColor.hasColor();