fix(android): NPE fix

Author: farfromrefug

packages/collectionview/platforms/android/java/com/nativescript/collectionview/GridLayoutManager.java

@@ -28,6 +28,9 @@ public  int computeVerticalScrollOffset(RecyclerView.State state) {
         }
         final int firstChildPosition = findFirstVisibleItemPosition();
         final View firstChild = findViewByPosition(firstChildPosition);
+        if (firstChild == null) {
+            return 0;
+        }
         float scrolledY = -firstChild.getY();
         for (int i = 0; i < firstChildPosition; i++) {
             if (childSizesMap.get(i) != null) {

packages/collectionview/platforms/android/java/com/nativescript/collectionview/PreCachingGridLayoutManager.java

@@ -33,6 +33,9 @@ public int computeVerticalScrollOffset(RecyclerView.State state) {
         // fixed computeVerticalScrollOffset
         View firstItemView = getChildAt(0);
         View lastItemView = getChildAt(getChildCount() - 1);
+        if (firstItemView == null || lastItemView == null) {
+            return 0;
+        }
 
         int firstItem = getPosition(firstItemView);
         int lastItem = getPosition(lastItemView);