Handle fling in collapsing header with image view (#1293)

fixes #1284 #1248

android/app/src/main/java/com/reactnativenavigation/views/collapsingToolbar/CollapsingTopBar.java

 import android.content.Context;
 import android.content.res.TypedArray;
 import android.os.Bundle;
+import android.support.annotation.Nullable;
 import android.view.View;
 import android.view.ViewGroup;
 import android.widget.ScrollView;
 
 public class CollapsingTopBar extends TopBar implements CollapsingView {
     private CollapsingTopBarBackground collapsingTopBarBackground;
-    private CollapsingTopBarReactHeader header;
+    private @Nullable CollapsingTopBarReactHeader header;
     private ScrollListener scrollListener;
     private float finalCollapsedTranslation;
     private final StyleParams styleParams;
     @Override
     public void destroy() {
         if (params.hasReactView()) {
+            assert header != null;
             header.unmountReactView();
         }
     }
                         @Override
                         public void onDisplay() {
                             calculateFinalCollapsedTranslation();
+                            assert header != null;
                             header.setOnDisplayListener(null);
                         }
                     });
     @Override
     public void fling(CollapseAmount amount) {
         if (titleBar instanceof CollapsingTitleBar) {
-            viewCollapser.fling(amount, (CollapsingTitleBar) titleBar, header);
+            if (header != null) {
+                viewCollapser.fling(amount, (CollapsingTitleBar) titleBar, header);
+            } else {
+                viewCollapser.fling(amount, (CollapsingTitleBar) titleBar);
+            }
         } else {
             viewCollapser.collapse(amount);
         }

android/app/src/main/java/com/reactnativenavigation/views/collapsingToolbar/ViewCollapser.java

         animator.start();
     }
 
-    void fling(final CollapseAmount amount, final CollapsingTitleBar titleBar, final CollapsingTopBarReactHeader header) {
+    void fling(final CollapseAmount amount, final CollapsingTitleBar titleBar, @NonNull final CollapsingTopBarReactHeader header) {
         fling(amount, new ValueAnimator.AnimatorUpdateListener() {
             @Override
             public void onAnimationUpdate(ValueAnimator animation) {
         });
     }
 
+    void fling(final CollapseAmount amount, final CollapsingTitleBar titleBar) {
+        fling(amount, new ValueAnimator.AnimatorUpdateListener() {
+            @Override
+            public void onAnimationUpdate(ValueAnimator animation) {
+                titleBar.collapse(new CollapseAmount((Float) animation.getAnimatedValue()));
+            }
+        });
+    }
+
     public void fling(CollapseAmount amount) {
         fling(amount, LISTENER);
     }