VectorTileLayer: check tile source on detach by @Longri, fixes #175

vtm/src/org/oscim/layers/tile/vector/VectorTileLayer.java

@@ -1,5 +1,6 @@
 /*
  * Copyright 2013 Hannes Janetzek
+ * Copyright 2016 Longri
  *
  * This file is part of the OpenScienceMap project (http://www.opensciencemap.org).
  *
@@ -175,7 +176,8 @@ public class VectorTileLayer extends TileLayer {
     @Override
     public void onDetach() {
         super.onDetach();
-        mTileSource.close();
+        if (mTileSource != null)
+            mTileSource.close();
     }
 
     public void callThemeHooks(MapTile tile, RenderBuckets layers, MapElement element,