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>