fix symbol:variable cursor

Author: maher-linkdev

android/src/main/java/flutter/plugins/contactsservice/contactsservice/ContactsServicePlugin.java

@@ -296,7 +296,9 @@ public boolean onActivityResult(int requestCode, int resultCode, Intent intent)
           Log.e(LOG_TAG, "onActivityResult - cursor.moveToFirst() returns false");
           finishWithResult(FORM_OPERATION_CANCELED);
         }}else{return true;}
-        cursor.close();
+        if (cursor != null) {
+          cursor.close();
+        }
         return true;
       }