element ui通过this.$refs.tree.getCheckedKeys()方法获取不到

Question

element ui在dialog对话框中使用tree组件，点击确定，通过`this.$refs.tree.getCheckedKeys()为什么获取不到值？该怎么解决？求大神！报错："TypeError: Cannot read property 'getCheckedKeys' of undefined"    <el-dialog title="分配角色" :visible.sync="showRoleTreeDialog">        <el-tree :data="roleTreeList" show-checkbox node-key="id" :props="{label: 'name'}"                 :default-checked-keys="checkedRoleIds" style="border: 0">        </el-tree>        <div slot="footer" class="dialog-footer">            <el-button @click="showRoleTreeDialog = false">取 消</el-button>            <el-button type="primary" @click="doAuthorize">确 定</el-button>        </div>    </el-dialog>