Correct SqlResultProvider::fetchField() to work with empty results.

model/SqlResultProvider.php

@@ -76,7 +76,11 @@ class SqlResultProvider
      */
     public function fetchField($field)
     {
-        return current($this->result_items)->{$field};
+        $item = current($this->result_items);
+        if ($item) {
+            return $item->{$field};
+        }
+        return false;
     }
 
     public function fetch()