TypeScript: Object is possibly 'null'. window.open

Question

How does one make this work in Typescript?

window.open(externalUrl, '_blank').focus();

this throws the following typescript error:

Object is possibly 'null'.ts(2531)

I have tried this, but it does not work:

if (typeof window != 'undefined' && window && externalUrl !== '' && window.open(externalUrl, '_blank') ) {
      window.open(externalUrl, '_blank').focus();
    }

Answer 1

Based on @jcalz's excellent answer, there is also one-liner, using !:

window.open(externalUrl, '_blank')!.focus();