fixed notice when remove column from grid

Author: magekey

app/code/Magento/Backend/Block/Widget/Grid/Extended.php

@@ -314,7 +314,8 @@ public function removeColumn($columnId)
         if ($this->getColumnSet()->getChildBlock($columnId)) {
             $this->getColumnSet()->unsetChild($columnId);
             if ($this->_lastColumnId == $columnId) {
-                $this->_lastColumnId = array_pop($this->getColumnSet()->getChildNames());
+                $names = $this->getColumnSet()->getChildNames();
+                $this->_lastColumnId = array_pop($names);
             }
         }
         return $this;