Fix fab (#458)

* Only destroy CollapsingContentView

* Fix Fab

android/app/src/main/java/com/reactnativenavigation/screens/CollapsingViewPagerScreen.java

@@ -101,7 +101,9 @@ public class CollapsingViewPagerScreen extends ViewPagerScreen {
     public void destroy() {
         super.destroy();
         for (ContentView contentView : contentViews) {
-            ((CollapsingContentView) contentView).destroy();
+            if (contentView instanceof CollapsingContentView) {
+                ((CollapsingContentView) contentView).destroy();
+            }
         }
     }
 }

src/deprecated/platformSpecificDeprecated.android.js

@@ -398,8 +398,8 @@ function addNavigatorButtons(screen, sideMenuParams) {
 }
 
 function getFab(screen) {
-  if (screen.fab) {
-    const fab = screen.fab;
+  if (screen.navigatorButtons && screen.navigatorButtons.fab) {
+    const fab = screen.navigatorButtons.fab;
     const collapsedIconUri = resolveAssetSource(fab.collapsedIcon);
     if (!collapsedIconUri) {
       return;