Prevent potential NPE

Ignore scroll events if mNestedScrollingChildRef is not set yet.
It happens if BottomSheetBehavior view is inflated from a ViewStub for
example, input events can arrive too soon.

bug: https://code.google.com/p/android/issues/detail?id=232646

Test: nullity check, existing unit tests still pass.
Change-Id: I25847ce70437d33b864b0d1b27a28d215a7c1ab3
diff --git a/design/src/android/support/design/widget/BottomSheetBehavior.java b/design/src/android/support/design/widget/BottomSheetBehavior.java
index e36e2ab..77563a9 100644
--- a/design/src/android/support/design/widget/BottomSheetBehavior.java
+++ b/design/src/android/support/design/widget/BottomSheetBehavior.java
@@ -385,7 +385,8 @@
             setStateInternal(STATE_EXPANDED);
             return;
         }
-        if (target != mNestedScrollingChildRef.get() || !mNestedScrolled) {
+        if (mNestedScrollingChildRef == null || target != mNestedScrollingChildRef.get()
+                || !mNestedScrolled) {
             return;
         }
         int top;