Single exclamation mark in Kotlin

It's the notation for platform types:

T! means "T or T?"

They're called platform types and they mean that Kotlin doesn't know whether that value can or cannot be null and it's up to you to decide if it's nullable or not.

In a nutshell, the problem is that any reference coming from Java may be null, and Kotlin, being null-safe by design, forced the user to null-check every Java value, or use safe calls (?.) or not-null assertions (!!). Those being very handy features in the pure Kotlin world, tend to turn into a disaster when you have to use them too often in the Kotlin/Java setting.

This is why we took a radical approach and made Kotlin’s type system more relaxed when it comes to Java interop: now references coming from Java have specially marked types -- Kotlin Blog