MapScaleBarLayer: check if scale bar is destroyed, fix #611

vtm/src/org/oscim/scalebar/MapScaleBarLayer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 devemux86
+ * Copyright 2016-2018 devemux86
  *
  * This program is free software: you can redistribute it and/or modify it under the
  * terms of the GNU Lesser General Public License as published by the Free Software
@@ -42,6 +42,9 @@ public class MapScaleBarLayer extends Layer implements Map.UpdateListener {
         if (e == Map.UPDATE_EVENT)
             return;
 
+        if (mapScaleBar.mapScaleBitmap == null)
+            return;
+
         if (!mapScaleBar.isVisible())
             return;