Language-tools: 然后等待，没有变量

描述错误
使用{：then}（不带变量）时出现错误Cannot read property 'end' of null 。

重现

<script>
  let promise;
</script>

{#await promise}
  Loading...
{:then}
  Loaded!
{/await}

预期行为
没错

屏幕截图

额外的背景
输出：

Error walking node  {
  start: 35,
  end: 83,
  type: 'AwaitBlock',
  expression: Node {
    type: 'Identifier',
    start: 43,
    end: 50,
    loc: SourceLocation { start: [Position], end: [Position] },
    name: 'promise'
  },
  value: null,
  error: null,
  pending: {
    start: 51,
    end: 61,
    type: 'PendingBlock',
    children: [ [Object] ],
    skip: false
  },
  then: {
    start: 61,
    end: 75,
    type: 'ThenBlock',
    children: [ [Object] ],
    skip: false
  },
  catch: {
    start: null,
    end: null,
    type: 'CatchBlock',
    children: [],
    skip: true
  }
}