Fixes NPE when FlexboxLayout doesn't have any children.

flexbox/src/androidTest/java/com/google/android/flexbox/test/FlexboxAndroidTest.java

@@ -2834,6 +2834,21 @@ public class FlexboxAndroidTest {
         onView(withId(R.id.text3)).check(isBottomAlignedWith(withId(R.id.flexbox_layout)));
     }
 
+    @Test
+    @FlakyTest(tolerance = TOLERANCE)
+    public void testEmptyChildren() throws Throwable {
+        final FlexboxTestActivity activity = mActivityRule.getActivity();
+        mActivityRule.runOnUiThread(new Runnable() {
+            @Override
+            public void run() {
+                activity.setContentView(R.layout.activity_empty_children);
+            }
+        });
+        FlexboxLayout flexboxLayout = (FlexboxLayout) activity.findViewById(R.id.flexbox_layout);
+
+        assertThat(flexboxLayout.getChildCount(), is(0));
+    }
+
     private TextView createTextView(Context context, String text, int order) {
         TextView textView = new TextView(context);
         textView.setText(text);

flexbox/src/androidTest/res/layout/activity_empty_children.xml

+<?xml version="1.0" encoding="utf-8"?><!--
+  Copyright 2016 Google Inc. All rights reserved.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+  -->
+<com.google.android.flexbox.FlexboxLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
+    android:id="@+id/flexbox_layout"
+    android:layout_width="320dp"
+    android:layout_height="320dp"
+    app:flexWrap="wrap">
+
+</com.google.android.flexbox.FlexboxLayout>
\ No newline at end of file

flexbox/src/main/java/com/google/android/flexbox/FlexboxLayout.java

@@ -400,6 +400,9 @@ public class FlexboxLayout extends ViewGroup {
      */
     private boolean isOrderChangedFromLastMeasurement() {
         int childCount = getChildCount();
+        if (mOrderCache == null) {
+            mOrderCache = new SparseIntArray(childCount);
+        }
         if (mOrderCache.size() != childCount) {
             return true;
         }