Update SDLActivity.java

Added a null check to fix the deck downloader menu option.

projects/mtg/Android/src/org/libsdl/app/SDLActivity.java

@@ -309,6 +309,7 @@ public class SDLActivity extends Activity implements OnKeyListener {
         File root = new File(System.getenv("EXTERNAL_STORAGE") + "/Download");
 	File[] files = root.listFiles();
 
+        if (files != null) {
 			for (File f : files) {
 				if (!myresult.contains(f.toString()) &&
 						(f.toString().contains(".txt") ||
@@ -317,6 +318,7 @@ public class SDLActivity extends Activity implements OnKeyListener {
 					myresult.add(f.toString());
 				}
 			}
+		}  
 
         //get first item?
         if (!myresult.isEmpty()) {