title: TypeScript Type 'string | null' is not assignable to type 'string'
body: <p>In this fragment I Have the next error :</p>
  <pre><code>Type 'string | null' is not assignable to type 'string'.
    Type 'null' is not assignable to type 'string'.  TS2322
  </code></pre>
  <pre><code>export async function GetCertainCoinByType(coinId: string) {
    const response = await axios.get(URLofCertainCoins + `certain/${coinId}`, {
        headers : {
            token : localStorage.getItem('token'),
        }
    });
    return response;
  }
  </code></pre>