Set window property in karma config

I have a webpack typescript karma jasmine setup. The webpack entry-point is main.ts. In there i am calling

window.addEventListener('resize', () => {    [...]});

If i run the application it is fine.

If i run the tests with karma i receive this error:

Error during loading: Uncaught TypeError: Cannot read property 'addEventListener' of null

I don't know how to fix that. Grateful for any help!