Smart cast to kotlin.String

The compiler knows that mTripStatus cannot be null if the if condition is satisfied, so it performs a smart cast from String? to String. That's what allows html.replace("TRIP_STATUS_VALUE", mTripStatus) to compile.

But note that this shouldn't be interpreted as a compiler warning. This is idiomatic Kotlin code.

This code:

var html :String = HTML
html = if (mTripStatus!=null) html.replace("TRIP_STATUS_VALUE", mTripStatus) else html

can be:

var html: String = html
mTripStatus?.let { html = html.replace("TRIP_STATUS_VALUE", mTripStatus) }