Fix possible NPE when setting nav bar button color

android/app/src/main/java/com/reactnativenavigation/views/RnnToolBar.java

 
         if (navArrow != null) {
             navArrow.setProgress(isBack ? 1.0f : 0.0f);
-            if (screen.navBarButtonColor != null) {
+            if (screen != null && screen.navBarButtonColor != null) {
                 navArrow.setColor(screen.navBarButtonColor);
             } else {
                 navArrow.setColor(Color.BLACK);