Android toolbars and whether they are nullable - android-layout

Question

As seems to be typical for Android development, it doesn't work.
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val toolbar: Toolbar = findViewById(R.id.toolbar)
//setSupportActionBar(toolbar) Why doesn't this work? Type mismatch: inferred type is android.widget.Toolbar but androidx.appcompat.widget.Toolbar? was expected
setSupportActionBar(findViewById(R.id.toolbar))
}
}
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >
<androidx.appcompat.widget.Toolbar
android:id="#+id/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="?attr/colorPrimary"
android:elevation="4dp"
android:theme="#style/ThemeOverlay.AppCompat.ActionBar"
app:popupTheme="#style/ThemeOverlay.AppCompat.Light"
/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</LinearLayout>
However:
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.rwb.toolbarhide, PID: 3989
java.lang.NoClassDefFoundError: android.widget.Toolbar
at com.rwb.toolbarhide.MainActivity.onCreate(MainActivity.kt:13)
at android.app.Activity.performCreate(Activity.java:5231)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5017)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
at dalvik.system.NativeStart.main(Native Method)
Is this something to do with androidx and different Toolbars?
How can you tell if this is so?
What can be done to fix it?
What I wanted to know, however, was why the line commented out doesn't work.
How can we tell if findViewById is returning a Toolbar or a Toolbar? (in VisualStudio you would just hover the mouse, but not so in AndroidStudio -- it seems to be impossible).
So, if indeed we do have a Toolbar and not a Toolbar? then how can we pass it to setSupportActionBar?
(I want a Toolbar not a Toolbar? because it's a right nuisance having to ? and !! all over the place.)

Answer 1

import android.widget.Toolbar. It would have been done by the intellisense if it wasn't there as standard because I have no idea what I'm doing. Did it guess the wrong one?
This seems to have pacified it:
import androidx.appcompat.widget.Toolbar