TypeScript Object is possibly 'null' error

[probablykasper]

Describe the bug
The following code:

<script lang="ts">
  type User =  { name: any } | null
  let user: User = {
    name: new Promise((resolve) => {
      setTimeout(() => {
        resolve('probablykasper')
      }, 1000)
    })
  }
</script>

{#if user}
  {#await user.name}
    Loading username
  {:then name}
    Username: {name}
  {/await}
{/if}

results in this TypeScript error:

To Reproduce
Repo: https://github.com/probablykasper/svelte-typescript-possibly-null-bug

Expected behavior
When await user.name is inside an if user block, user cannot be null, so there should not be any TypeScript warning

System (please complete the following information):

• OS: macOS 10.14.6
• IDE: VSCode
• Plugin/Package: Svelte for VSCode, svelte-check

[probablykasper]

Similar issues: #619, #493