getFragmentManager() not working in Kotlin

Since your searchDialogFragment variable is marked as nullable with the question mark in the declaration you need to use the safe call operator ?.. It only executes when searchDialogFragment is not null:

searchDialogFragment?.show(supportFragmentManager, null)

Or you could use the following to declare your variable as not null but still be able to initialize it later in your code:

private lateinit var searchDialogFragment: SearchDialogFragment