Fixed #128 - Behavior for which getStickySectionHeadersHolder() shoul…

…d return a ViewGroup with LayoutParams coherent and initialized by the user who knows how these params has to be initialized. An exception will be raised if params are nulls.

flexible-adapter/src/main/java/eu/davidea/flexibleadapter/helpers/StickyHeaderHelper.java

@@ -111,6 +111,9 @@ private void initStickyHeadersHolder() {
 		//Initialize Holder Layout and show sticky header if exists already
 		mStickyHolderLayout = mAdapter.getStickySectionHeadersHolder();
 		if (mStickyHolderLayout != null) {
+			if (mStickyHolderLayout.getLayoutParams() == null) {
+				throw new IllegalStateException("The ViewGroup provided, doesn't have LayoutParams correctly set, please initialize the ViewGroup accordingly");
+			}
 			mStickyHolderLayout.setClipToPadding(false);
 			mStickyHolderLayout.setAlpha(0);
 			updateOrClearHeader(false);