Fixed issue #474 (#490)

* Fixed issue 474, canScrollHorizontally() throws NPE if the RecyclerView is not attached to the Window

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

@@ -1898,7 +1898,7 @@ public class FlexboxLayoutManager extends RecyclerView.LayoutManager implements
         if (mFlexWrap == FlexWrap.NOWRAP) {
             return isMainAxisDirectionHorizontal();
         } else {
-            return !isMainAxisDirectionHorizontal() || getWidth() > mParent.getWidth();
+            return !isMainAxisDirectionHorizontal() || getWidth() > (mParent != null ? mParent.getWidth() : 0);
         }
     }
 
@@ -1907,7 +1907,7 @@ public class FlexboxLayoutManager extends RecyclerView.LayoutManager implements
         if (mFlexWrap == FlexWrap.NOWRAP) {
             return !isMainAxisDirectionHorizontal();
         } else {
-            return isMainAxisDirectionHorizontal() || getHeight() > mParent.getHeight();
+            return isMainAxisDirectionHorizontal() || getHeight() > (mParent != null ? mParent.getHeight() : 0);
         }
     }