button crushing app (Kotlin)

Iam just starting to learn programming and I am still in my first app it is an age calculator here is the code that runs the app:

here is the user interface: https://imgur.com/a/n7MsY

The problem is when the button is clicked without a user input the app crushes , how do I fix it.

also I want to add some calculations for days I want to get a value out of If statement , how can I do it?