fix NullPointerException

Author: MaximBelov

src/android/SMSRetriever.java

@@ -256,7 +256,9 @@ public void onActivityResult(int requestCode, int resultCode, Intent intent) {
             if(ex instanceof ApiException){
                 getPhoneNumberCallbackContext.error(ex.getMessage());
             }
-            ex.printStackTrace();
+            if(ex != null){
+                ex.printStackTrace();
+            }
         }
     }